diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index c800b71..9ae8e22 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -15,6 +15,8 @@ Small multi-service digital signage app (Flask API, React dashboard, MQTT schedu
 - "Refactor `scheduler/db_utils.py` to prefer precomputed EventMedia metadata and fall back to a HEAD probe." 
 - "Add an ffprobe-based worker that extracts duration/resolution/bitrate and stores them on `EventMedia`."
 
+Keep docs synced with code. When you change services/MQTT/API/UTC/env or dev/prod run steps, update this file in the same commit (see `AI-INSTRUCTIONS-MAINTENANCE.md`).
+
 ### When not to change
 - Avoid editing generated assets under `dashboard/dist/` and compiled bundles. Don't modify files produced by CI or Docker builds (unless intentionally updating build outputs).
 
@@ -40,6 +42,8 @@ Small multi-service digital signage app (Flask API, React dashboard, MQTT schedu
   - Scheduler: Publishes only currently active events (per group, at "now") to MQTT retained topics in `scheduler/scheduler.py`. It queries a future window (default: 7 days) to expand recurring events using RFC 5545 rules and applies event exceptions, but only publishes events that are active at the current time. When a group has no active events, the scheduler clears its retained topic by publishing an empty list. All time comparisons are UTC; any naive timestamps are normalized. Logging is concise; conversion lookups are cached and logged only once per media.
   - Nginx: Reverse proxy routes `/api/*` and `/screenshots/*` to API; everything else to dashboard (`nginx.conf`).
 
+  - Dev Container (hygiene): UI-only `Dev Containers` extension runs on host UI via `remote.extensionKind`; do not install it in-container. Dashboard installs use `npm ci`; shell aliases in `postStartCommand` are appended idempotently.
+
   ## Recent changes since last commit
 
   ### Latest (November 2025)
@@ -57,6 +61,7 @@ Small multi-service digital signage app (Flask API, React dashboard, MQTT schedu
     - Frontend: Dashboard and appointments automatically append 'Z' to parse as UTC and display in user's local timezone
     - Time formatting functions use `toLocaleTimeString('de-DE')` for German locale display
     - All time comparisons use UTC; `new Date().toISOString()` sends UTC back to API
+    - API returns ISO strings without `Z`; frontend must append `Z` before parsing to ensure UTC
 
   - **Dashboard Enhancements**:
     - New card-based design for Raumgruppen (room groups) with Syncfusion components
@@ -84,6 +89,11 @@ Small multi-service digital signage app (Flask API, React dashboard, MQTT schedu
 
   Note: these edits are intentionally backwards-compatible — if the probe fails, the scheduler still emits the stream URL and the client should fallback to a direct play attempt or request richer metadata when available.
 
+  Backend rework notes (no version bump):
+  - Dev container hygiene: UI-only Remote Containers; reproducible dashboard installs (`npm ci`); idempotent shell aliases.
+  - Serialization consistency: snake_case internal → camelCase external via `server/serializers.py` for all JSON.
+  - UTC normalization across routes/scheduler; enums and datetimes serialize consistently.
+
 ## Service boundaries & data flow
 - Database connection string is passed as `DB_CONN` (mysql+pymysql) to Python services.
   - API builds its engine in `server/database.py` (loads `.env` only in development).
@@ -96,6 +106,8 @@ Small multi-service digital signage app (Flask API, React dashboard, MQTT schedu
   - Per-client group assignment (retained): `infoscreen/{uuid}/group_id` via `server/mqtt_helper.py`.
 - Screenshots: server-side folders `server/received_screenshots/` and `server/screenshots/`; Nginx exposes `/screenshots/{uuid}.jpg` via `server/wsgi.py` route.
 
+- Dev Container guidance: If extensions reappear inside the container, remove UI-only extensions from `devcontainer.json` `extensions` and map them in `remote.extensionKind` as `"ui"`.
+
 - Presentation conversion (PPT/PPTX/ODP → PDF):
   - Trigger: on upload in `server/routes/eventmedia.py` for media types `ppt|pptx|odp` (compute sha256, upsert `Conversion`, enqueue job).
   - Worker: RQ worker runs `server.worker.convert_event_media_to_pdf`, calls Gotenberg LibreOffice endpoint, writes to `server/media/converted/`.
@@ -141,12 +153,16 @@ Small multi-service digital signage app (Flask API, React dashboard, MQTT schedu
     - `POST /api/academic_periods/active` — set active period (deactivates others)
     - `GET /api/academic_periods/for_date?date=YYYY-MM-DD` — period covering given date
 
+  Documentation maintenance: keep this file aligned with real patterns; update when routes/session/UTC rules change. Avoid long prose; link exact paths.
+
 ## Frontend patterns (dashboard)
 - Vite React app; proxies `/api` and `/screenshots` to API in dev (`vite.config.ts`).
 - Uses Syncfusion components; Vite config pre-bundles specific packages to avoid alias issues.
 - Environment: `VITE_API_URL` provided at build/run; in dev compose, proxy handles `/api` so local fetches can use relative `/api/...` paths.
 - **API Response Format**: All API endpoints return camelCase JSON (e.g., `startTime`, `endTime`, `groupId`). Frontend consumes camelCase directly.
 - **UTC Time Parsing**: API returns ISO strings without 'Z' suffix. Frontend appends 'Z' before parsing to ensure UTC interpretation: `const utcString = dateStr.endsWith('Z') ? dateStr : dateStr + 'Z'; new Date(utcString);`. Display uses `toLocaleTimeString('de-DE')` for German format.
+
+- Dev Container: When adding frontend deps, prefer `npm ci` and, if using named volumes, recreate dashboard `node_modules` volume so installs occur inside the container.
 - Theming: Syncfusion Material 3 theme is used. All component CSS is imported centrally in `dashboard/src/main.tsx` (base, navigations, buttons, inputs, dropdowns, popups, kanban, grids, schedule, filemanager, notifications, layouts, lists, calendars, splitbuttons, icons). Tailwind CSS has been removed.
  - Scheduler (appointments page): top bar includes Group and Academic Period selectors (Syncfusion DropDownList). Selecting a period calls `POST /api/academic_periods/active`, moves the calendar to today’s month/day within the period year, and refreshes a right-aligned indicator row showing:
    - Holidays present in the current view (count)
@@ -252,6 +268,7 @@ Note: Syncfusion usage in the dashboard is already documented above; if a UI for
   - Frontend **must** append 'Z' before parsing: `const utcStr = dateStr.endsWith('Z') ? dateStr : dateStr + 'Z'; new Date(utcStr);`
   - Display in local timezone using `toLocaleTimeString('de-DE', { hour: '2-digit', minute: '2-digit' })`
   - When sending to API, use `date.toISOString()` which includes 'Z' and is UTC
+  - Frontend must append `Z` to API strings before parsing; backend compares in UTC and returns ISO without `Z`.
 - **JSON Naming Convention**:
   - Backend uses snake_case internally (Python convention)
   - API returns camelCase JSON (web standard): `startTime`, `endTime`, `groupId`, etc.
@@ -268,6 +285,8 @@ Note: Syncfusion usage in the dashboard is already documented above; if a UI for
   3) Manage `Session()` lifecycle,
   4) Return JSON-safe values (serialize enums and datetimes), and
   5) Use `dict_to_camel_case()` for camelCase JSON responses
+
+Docs maintenance guardrails (solo-friendly): Update this file alongside code changes (services/MQTT/API/UTC/env). Keep it concise (20–50 lines per section). Never include secrets.
 - When extending media types, update `MediaType` and any logic in `eventmedia` and dashboard that depends on it.
 - Academic periods: Events/media can be optionally associated with periods for educational organization. Only one period should be active at a time (`is_active=True`).
 - Initialization scripts: legacy DB init scripts were removed; use Alembic and `initialize_database.py` going forward.
diff --git a/README.md b/README.md
index f2a434b..0930664 100644
--- a/README.md
+++ b/README.md
@@ -217,6 +217,7 @@ For detailed deployment instructions, see:
  - Videos include a `video` payload with a stream URL and playback flags:
    - `video`: includes `url` (streaming endpoint) and `autoplay`, `loop`, `volume`, `muted`
    - Streaming endpoint supports byte-range requests (206) to enable seeking: `/api/eventmedia/stream/<media_id>/<filename>`
+   - Server-side UTC: All backend comparisons are performed in UTC; API returns ISO strings without `Z`. Frontend appends `Z` before parsing.
   
 ## Recent changes since last commit
 
@@ -232,6 +233,18 @@ These changes are designed to be safe if metadata extraction or probes fail —
 See `MQTT_EVENT_PAYLOAD_GUIDE.md` for details.
   - `infoscreen/{uuid}/group_id` - Client group assignment
 
+## 🧩 Developer Environment Notes (Dev Container)
+- Extensions: UI-only `Dev Containers` runs on the host UI; not installed inside the container to avoid reinstallation loops. See `/.devcontainer/devcontainer.json` (`remote.extensionKind`).
+- Installs: Dashboard uses `npm ci` on `postCreateCommand` for reproducible installs.
+- Aliases: `postStartCommand` appends shell aliases idempotently to prevent duplicates across restarts.
+
+## 📦 Versioning
+- Unified app version: Use a single SemVer for the product (e.g., `2025.1.0-beta.3`) — simplest for users and release management.
+- Pre-releases: Use identifiers like `-alpha.N`, `-beta.N`, `-rc.N` for stage tracking.
+- Build metadata: Optionally include component build info (non-ordering) e.g., `+api.abcd123,dash.efgh456,sch.jkl789,wkr.mno012`.
+- Component traceability: Document component SHAs or image tags under each TECH-CHANGELOG release entry rather than exposing separate user-facing versions.
+- Hotfixes: For backend-only fixes, prefer a patch bump or pre-release increment, and record component metadata under the unified version.
+
 ## 📁 Project Structure
 
 ```
diff --git a/TECH-CHANGELOG.md b/TECH-CHANGELOG.md
index f99f688..05a9174 100644
--- a/TECH-CHANGELOG.md
+++ b/TECH-CHANGELOG.md
@@ -8,6 +8,13 @@ This changelog documents technical and developer-relevant changes included in pu
 ---
 
 ## 2025.1.0-alpha.12 (2025-11-27)
+Backend rework (post-release notes; no version bump):
+- 🧩 Dev Container hygiene: Remote Containers runs on UI (`remote.extensionKind`), removed in-container install to prevent reappearance loops; switched `postCreateCommand` to `npm ci` for reproducible dashboard installs; `postStartCommand` aliases made idempotent.
+- 🔄 Serialization: Consolidated snake_case→camelCase via `server/serializers.py` for all JSON outputs; ensured enums/UTC datetimes serialize consistently across routes.
+- 🕒 Time handling: Normalized naive timestamps to UTC in all back-end comparisons (events, scheduler, groups) and kept ISO strings without `Z` in API responses; frontend appends `Z`.
+- 📡 Streaming: Stabilized range-capable endpoint (`/api/eventmedia/stream/<media_id>/<filename>`), clarified client handling; scheduler emits basic HEAD-probe metadata (`mime_type`, `size`, `accept_ranges`).
+- 📅 Recurrence/exceptions: Ensured EXDATE tokens (RFC 5545 UTC) align with occurrence start; detached-occurrence flow confirmed via `POST /api/events/<id>/occurrences/<date>/detach`.
+- 🧰 Routes cleanup: Applied `dict_to_camel_case()` before `jsonify()` uniformly; verified Session lifecycle consistency (open/commit/close) across blueprints.
 - 🔄 **API Naming Convention Standardization**:
 	- Created `server/serializers.py` with `dict_to_camel_case()` and `dict_to_snake_case()` utilities for consistent JSON serialization
 	- Events API refactored: `GET /api/events` and `GET /api/events/<id>` now return camelCase JSON (`id`, `subject`, `startTime`, `endTime`, `type`, `groupId`, etc.) instead of PascalCase
@@ -46,6 +53,23 @@ Notes for integrators:
 - **Breaking change**: All Events API endpoints now return camelCase field names. Update client code accordingly.
 - Frontend must append 'Z' to API datetime strings before parsing: `const utcStr = dateStr.endsWith('Z') ? dateStr : dateStr + 'Z'; new Date(utcStr);`
 - Use `dict_to_camel_case()` from `server/serializers.py` for any new API endpoints returning JSON
+ - Dev container: prefer `npm ci` and UI-only Remote Containers to avoid extension drift in-container.
+
+---
+
+### Component build metadata template (for traceability)
+Record component builds under the unified app version when releasing:
+
+```
+Component builds for this release
+- API: image tag `ghcr.io/robbstarkaustria/api:<short-sha>` (commit `<sha>`)
+- Dashboard: image tag `ghcr.io/robbstarkaustria/dashboard:<short-sha>` (commit `<sha>`)
+- Scheduler: image tag `ghcr.io/robbstarkaustria/scheduler:<short-sha>` (commit `<sha>`)
+- Listener: image tag `ghcr.io/robbstarkaustria/listener:<short-sha>` (commit `<sha>`)
+- Worker: image tag `ghcr.io/robbstarkaustria/worker:<short-sha>` (commit `<sha>`)
+```
+
+This is informational (build metadata) and does not change the user-facing version number.
 
 ## 2025.1.0-alpha.11 (2025-11-05)
 - 🗃️ Data model & API:
diff --git a/dashboard/package-lock.json b/dashboard/package-lock.json
index 8ddbfa1..5b97963 100644
--- a/dashboard/package-lock.json
+++ b/dashboard/package-lock.json
@@ -98,6 +98,7 @@
       "integrity": "sha512-2BCOP7TN8M+gVDj7/ht3hsaO/B/n5oDbiAyyvnRlNOs+u1o+JWNYTQrmpuNp1/Wq2gcFrI01JAW+paEKDMx/CA==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "@babel/code-frame": "^7.27.1",
         "@babel/generator": "^7.28.3",
@@ -385,6 +386,7 @@
         }
       ],
       "license": "MIT",
+      "peer": true,
       "engines": {
         "node": ">=18"
       },
@@ -408,6 +410,7 @@
         }
       ],
       "license": "MIT",
+      "peer": true,
       "engines": {
         "node": ">=18"
       }
@@ -2052,6 +2055,7 @@
       "integrity": "sha512-cMoR+FoAf/Jyq6+Df2/Z41jISvGZZ2eTlnsaJRptmZ76Caldwy1odD4xTr/gNV9VLj0AWgg/nmkevIyUfIIq5w==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "csstype": "^3.0.2"
       }
@@ -2125,6 +2129,7 @@
       "integrity": "sha512-B7RIQiTsCBBmY+yW4+ILd6mF5h1FUwJsVvpqkrgpszYifetQ2Ke+Z4u6aZh0CblkUGIdR59iYVyXqqZGkZ3aBw==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "@typescript-eslint/scope-manager": "8.43.0",
         "@typescript-eslint/types": "8.43.0",
@@ -2357,6 +2362,7 @@
       "integrity": "sha512-NZyJarBfL7nWwIq+FDL6Zp/yHEhePMNnnJ0y3qfieCrmNvYct8uvtiV41UvlSe6apAfk0fY1FbWx+NwfmpvtTg==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "bin": {
         "acorn": "bin/acorn"
       },
@@ -2712,6 +2718,7 @@
         }
       ],
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "caniuse-lite": "^1.0.30001737",
         "electron-to-chromium": "^1.5.211",
@@ -3475,6 +3482,7 @@
       "integrity": "sha512-QePbBFMJFjgmlE+cXAlbHZbHpdFVS2E/6vzCy7aKlebddvl1vadiC4JFV5u/wqTkNUwEV8WrQi257jf5f06hrg==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "@eslint-community/eslint-utils": "^4.8.0",
         "@eslint-community/regexpp": "^4.12.1",
@@ -3536,6 +3544,7 @@
       "integrity": "sha512-82GZUjRS0p/jganf6q1rEO25VSoHH0hKPCTrgillPjdI/3bgBhAE1QzHrHTizjpRvy6pGAvKjDJtk2pF9NDq8w==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "bin": {
         "eslint-config-prettier": "bin/cli.js"
       },
@@ -4990,18 +4999,6 @@
         "@pkgjs/parseargs": "^0.11.0"
       }
     },
-    "node_modules/jiti": {
-      "version": "1.21.7",
-      "resolved": "https://registry.npmjs.org/jiti/-/jiti-1.21.7.tgz",
-      "integrity": "sha512-/imKNG4EbWNrVjoNC/1H5/9GFy+tqjGBHCaSsN+P2RnPqjsLmv6UD3Ej+Kj8nBWaRAwyk7kK5ZUc+OEatnTR3A==",
-      "dev": true,
-      "license": "MIT",
-      "optional": true,
-      "peer": true,
-      "bin": {
-        "jiti": "bin/jiti.js"
-      }
-    },
     "node_modules/js-tokens": {
       "version": "4.0.0",
       "resolved": "https://registry.npmjs.org/js-tokens/-/js-tokens-4.0.0.tgz",
@@ -5717,6 +5714,7 @@
         }
       ],
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "nanoid": "^3.3.11",
         "picocolors": "^1.1.1",
@@ -5783,6 +5781,7 @@
       "integrity": "sha512-I7AIg5boAr5R0FFtJ6rCfD+LFsWHp81dolrFD8S79U9tb8Az2nGrJncnMSnys+bpQJfRUzqs9hnA81OAA3hCuQ==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "bin": {
         "prettier": "bin/prettier.cjs"
       },
@@ -5871,6 +5870,7 @@
       "resolved": "https://registry.npmjs.org/react/-/react-19.1.1.tgz",
       "integrity": "sha512-w8nqGImo45dmMIfljjMwOGtbmC/mk4CMYhWIicdSflH91J9TyCyczcPFXJzrZ/ZXcgGRFeP6BU0BEJTw6tZdfQ==",
       "license": "MIT",
+      "peer": true,
       "engines": {
         "node": ">=0.10.0"
       }
@@ -5880,6 +5880,7 @@
       "resolved": "https://registry.npmjs.org/react-dom/-/react-dom-19.1.1.tgz",
       "integrity": "sha512-Dlq/5LAZgF0Gaz6yiqZCf6VCcZs1ghAJyrsu84Q/GT0gV+mCxbfmKNoGRKBYMJ8IEdGPqu49YWXD02GCknEDkw==",
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "scheduler": "^0.26.0"
       },
@@ -6784,6 +6785,7 @@
       "integrity": "sha512-8sLjZwK0R+JlxlYcTuVnyT2v+htpdrjDOKuMcOVdYjt52Lh8hWRYpxBPoKx/Zg+bcjc3wx6fmQevMmUztS/ccA==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "cssesc": "^3.0.0",
         "util-deprecate": "^1.0.2"
@@ -7022,6 +7024,7 @@
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "engines": {
         "node": ">=12"
       },
@@ -7152,6 +7155,7 @@
       "integrity": "sha512-p1diW6TqL9L07nNxvRMM7hMMw4c5XOo/1ibL4aAIGmSAt9slTE1Xgw5KWuof2uTOvCg9BY7ZRi+GaF+7sfgPeQ==",
       "dev": true,
       "license": "Apache-2.0",
+      "peer": true,
       "bin": {
         "tsc": "bin/tsc",
         "tsserver": "bin/tsserver"
@@ -7257,6 +7261,7 @@
       "integrity": "sha512-0msEVHJEScQbhkbVTb/4iHZdJ6SXp/AvxL2sjwYQFfBqleHtnCqv1J3sa9zbWz/6kW1m9Tfzn92vW+kZ1WV6QA==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "dependencies": {
         "esbuild": "^0.25.0",
         "fdir": "^6.4.4",
@@ -7350,6 +7355,7 @@
       "integrity": "sha512-5gTmgEY/sqK6gFXLIsQNH19lWb4ebPDLA4SdLP7dsWkIXHWlG66oPuVvXSGFPppYZz8ZDZq0dYYrbHfBCVUb1Q==",
       "dev": true,
       "license": "MIT",
+      "peer": true,
       "engines": {
         "node": ">=12"
       },
@@ -7580,21 +7586,6 @@
       "dev": true,
       "license": "ISC"
     },
-    "node_modules/yaml": {
-      "version": "2.8.1",
-      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.8.1.tgz",
-      "integrity": "sha512-lcYcMxX2PO9XMGvAJkJ3OsNMw+/7FKes7/hgerGUYWIoWu5j/+YQqcZr5JnPZWzOsEBgMbSbiSTn/dv/69Mkpw==",
-      "dev": true,
-      "license": "ISC",
-      "optional": true,
-      "peer": true,
-      "bin": {
-        "yaml": "bin.mjs"
-      },
-      "engines": {
-        "node": ">= 14.6"
-      }
-    },
     "node_modules/yauzl": {
       "version": "2.10.0",
       "resolved": "https://registry.npmjs.org/yauzl/-/yauzl-2.10.0.tgz",
diff --git a/dashboard/src/apiSystemSettings.ts b/dashboard/src/apiSystemSettings.ts
index f0eb493..07da30b 100644
--- a/dashboard/src/apiSystemSettings.ts
+++ b/dashboard/src/apiSystemSettings.ts
@@ -106,3 +106,34 @@ export async function updateSupplementTableSettings(
   }
   return response.json();
 }
+
+/**
+ * Get holiday banner setting
+ */
+export async function getHolidayBannerSetting(): Promise<{ enabled: boolean }> {
+  const response = await fetch(`/api/system-settings/holiday-banner`, {
+    credentials: 'include',
+  });
+  if (!response.ok) {
+    throw new Error(`Failed to fetch holiday banner setting: ${response.statusText}`);
+  }
+  return response.json();
+}
+
+/**
+ * Update holiday banner setting
+ */
+export async function updateHolidayBannerSetting(
+  enabled: boolean
+): Promise<{ enabled: boolean; message: string }> {
+  const response = await fetch(`/api/system-settings/holiday-banner`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    credentials: 'include',
+    body: JSON.stringify({ enabled }),
+  });
+  if (!response.ok) {
+    throw new Error(`Failed to update holiday banner setting: ${response.statusText}`);
+  }
+  return response.json();
+}
diff --git a/dashboard/src/dashboard.tsx b/dashboard/src/dashboard.tsx
index 48cd898..a84c5ae 100644
--- a/dashboard/src/dashboard.tsx
+++ b/dashboard/src/dashboard.tsx
@@ -3,7 +3,10 @@ import { fetchGroupsWithClients, restartClient } from './apiClients';
 import type { Group, Client } from './apiClients';
 import { fetchEvents } from './apiEvents';
 import { ButtonComponent } from '@syncfusion/ej2-react-buttons';
-import { ToastComponent } from '@syncfusion/ej2-react-notifications';
+import { ToastComponent, MessageComponent } from '@syncfusion/ej2-react-notifications';
+import { listHolidays } from './apiHolidays';
+import { getActiveAcademicPeriod, type AcademicPeriod } from './apiAcademicPeriods';
+import { getHolidayBannerSetting } from './apiSystemSettings';
 
 const REFRESH_INTERVAL = 15000; // 15 Sekunden
 
@@ -31,6 +34,15 @@ const Dashboard: React.FC = () => {
   const [activeEvents, setActiveEvents] = useState<GroupEvents>({});
   const toastRef = React.useRef<ToastComponent>(null);
 
+  // Holiday status state
+  const [holidayBannerEnabled, setHolidayBannerEnabled] = useState<boolean>(true);
+  const [activePeriod, setActivePeriod] = useState<AcademicPeriod | null>(null);
+  const [holidayOverlapCount, setHolidayOverlapCount] = useState<number>(0);
+  const [holidayFirst, setHolidayFirst] = useState<string | null>(null);
+  const [holidayLast, setHolidayLast] = useState<string | null>(null);
+  const [holidayLoading, setHolidayLoading] = useState<boolean>(false);
+  const [holidayError, setHolidayError] = useState<string | null>(null);
+
   // Optimiertes Update: Nur bei echten Datenänderungen wird das Grid aktualisiert
   useEffect(() => {
     let lastGroups: Group[] = [];
@@ -72,6 +84,62 @@ const Dashboard: React.FC = () => {
     return () => clearInterval(interval);
   }, []);
 
+  // Load academic period & holidays status
+  useEffect(() => {
+    const loadHolidayStatus = async () => {
+      // Check if banner is enabled first
+      try {
+        const bannerSetting = await getHolidayBannerSetting();
+        setHolidayBannerEnabled(bannerSetting.enabled);
+        if (!bannerSetting.enabled) {
+          return; // Skip loading if disabled
+        }
+      } catch (e) {
+        console.error('Fehler beim Laden der Banner-Einstellung:', e);
+        // Continue with default (enabled)
+      }
+
+      setHolidayLoading(true);
+      setHolidayError(null);
+      try {
+        const period = await getActiveAcademicPeriod();
+        setActivePeriod(period || null);
+        const holidayData = await listHolidays();
+        const list = holidayData.holidays || [];
+        
+        if (period) {
+          // Check for holidays overlapping with active period
+          const ps = new Date(period.start_date + 'T00:00:00');
+          const pe = new Date(period.end_date + 'T23:59:59');
+          const overlapping = list.filter(h => {
+            const hs = new Date(h.start_date + 'T00:00:00');
+            const he = new Date(h.end_date + 'T23:59:59');
+            return hs <= pe && he >= ps;
+          });
+          setHolidayOverlapCount(overlapping.length);
+          if (overlapping.length > 0) {
+            const sorted = overlapping.slice().sort((a, b) => a.start_date.localeCompare(b.start_date));
+            setHolidayFirst(sorted[0].start_date);
+            setHolidayLast(sorted[sorted.length - 1].end_date);
+          } else {
+            setHolidayFirst(null);
+            setHolidayLast(null);
+          }
+        } else {
+          setHolidayOverlapCount(0);
+          setHolidayFirst(null);
+          setHolidayLast(null);
+        }
+      } catch (e) {
+        const msg = e instanceof Error ? e.message : 'Ferienstatus konnte nicht geladen werden';
+        setHolidayError(msg);
+      } finally {
+        setHolidayLoading(false);
+      }
+    };
+    loadHolidayStatus();
+  }, []);
+
   // Fetch currently active events for all groups
   const fetchActiveEventsForGroups = async (groupsList: Group[]) => {
     const now = new Date();
@@ -344,6 +412,55 @@ const Dashboard: React.FC = () => {
     });
   };
 
+  // Format date for holiday display
+  const formatDate = (iso: string | null) => {
+    if (!iso) return '-';
+    try {
+      const d = new Date(iso + 'T00:00:00');
+      return d.toLocaleDateString('de-DE');
+    } catch { return iso; }
+  };
+
+  // Holiday Status Banner Component
+  const HolidayStatusBanner = () => {
+    if (holidayLoading) {
+      return (
+        <MessageComponent severity="Info" variant="Filled">
+          Lade Ferienstatus ...
+        </MessageComponent>
+      );
+    }
+    if (holidayError) {
+      return (
+        <MessageComponent severity="Error" variant="Filled">
+          Fehler beim Laden des Ferienstatus: {holidayError}
+        </MessageComponent>
+      );
+    }
+    if (!activePeriod) {
+      return (
+        <MessageComponent severity="Warning" variant="Outlined">
+          ⚠️ Keine aktive akademische Periode gesetzt – Ferienplan nicht verknüpft.
+        </MessageComponent>
+      );
+    }
+    if (holidayOverlapCount > 0) {
+      return (
+        <MessageComponent severity="Success" variant="Filled">
+          ✅ Ferienplan vorhanden für <strong>{activePeriod.display_name || activePeriod.name}</strong>: {holidayOverlapCount} Zeitraum{holidayOverlapCount === 1 ? '' : 'e'}
+          {holidayFirst && holidayLast && (
+            <> ({formatDate(holidayFirst)} – {formatDate(holidayLast)})</>
+          )}
+        </MessageComponent>
+      );
+    }
+    return (
+      <MessageComponent severity="Warning" variant="Filled">
+        ⚠️ Kein Ferienplan für <strong>{activePeriod.display_name || activePeriod.name}</strong> importiert. Jetzt unter Einstellungen → 📅 Kalender hochladen.
+      </MessageComponent>
+    );
+  };
+
   return (
     <div>
       <ToastComponent
@@ -361,6 +478,13 @@ const Dashboard: React.FC = () => {
         </p>
       </header>
 
+      {/* Holiday Status Banner */}
+      {holidayBannerEnabled && (
+        <div style={{ marginBottom: '20px' }}>
+          <HolidayStatusBanner />
+        </div>
+      )}
+
       {/* Global Statistics Summary */}
       {(() => {
         const globalStats = getGlobalStats();
diff --git a/dashboard/src/settings.tsx b/dashboard/src/settings.tsx
index a84f4dd..94ceccd 100644
--- a/dashboard/src/settings.tsx
+++ b/dashboard/src/settings.tsx
@@ -4,7 +4,7 @@ import { NumericTextBoxComponent, TextBoxComponent } from '@syncfusion/ej2-react
 import { ButtonComponent, CheckBoxComponent } from '@syncfusion/ej2-react-buttons';
 import { ToastComponent } from '@syncfusion/ej2-react-notifications';
 import { listHolidays, uploadHolidaysCsv, type Holiday } from './apiHolidays';
-import { getSupplementTableSettings, updateSupplementTableSettings } from './apiSystemSettings';
+import { getSupplementTableSettings, updateSupplementTableSettings, getHolidayBannerSetting, updateHolidayBannerSetting } from './apiSystemSettings';
 import { useAuth } from './useAuth';
 import { DropDownListComponent } from '@syncfusion/ej2-react-dropdowns';
 import { listAcademicPeriods, getActiveAcademicPeriod, setActiveAcademicPeriod, type AcademicPeriod } from './apiAcademicPeriods';
@@ -77,6 +77,10 @@ const Einstellungen: React.FC = () => {
   const [supplementEnabled, setSupplementEnabled] = React.useState(false);
   const [supplementBusy, setSupplementBusy] = React.useState(false);
 
+  // Holiday banner state
+  const [holidayBannerEnabled, setHolidayBannerEnabled] = React.useState(true);
+  const [holidayBannerBusy, setHolidayBannerBusy] = React.useState(false);
+
   // Video defaults state (Admin+)
   const [videoAutoplay, setVideoAutoplay] = React.useState<boolean>(true);
   const [videoLoop, setVideoLoop] = React.useState<boolean>(true);
@@ -116,6 +120,15 @@ const Einstellungen: React.FC = () => {
     }
   }, []);
 
+  const loadHolidayBannerSetting = React.useCallback(async () => {
+    try {
+      const data = await getHolidayBannerSetting();
+      setHolidayBannerEnabled(data.enabled);
+    } catch (e) {
+      console.error('Fehler beim Laden der Ferienbanner-Einstellung:', e);
+    }
+  }, []);
+
   // Load video default settings (with fallbacks)
   const loadVideoSettings = React.useCallback(async () => {
     try {
@@ -156,6 +169,7 @@ const Einstellungen: React.FC = () => {
 
   React.useEffect(() => {
     refresh();
+    loadHolidayBannerSetting(); // Everyone can see this
     if (user) {
       // Academic periods for all users
       loadAcademicPeriods();
@@ -166,7 +180,7 @@ const Einstellungen: React.FC = () => {
         loadVideoSettings();
       }
     }
-  }, [refresh, loadSupplementSettings, loadAcademicPeriods, loadPresentationSettings, loadVideoSettings, user]);
+  }, [refresh, loadSupplementSettings, loadAcademicPeriods, loadPresentationSettings, loadVideoSettings, loadHolidayBannerSetting, user]);
 
   const onUpload = async () => {
     if (!file) return;
@@ -208,6 +222,19 @@ const Einstellungen: React.FC = () => {
     }
   };
 
+  const onSaveHolidayBannerSetting = async () => {
+    setHolidayBannerBusy(true);
+    try {
+      await updateHolidayBannerSetting(holidayBannerEnabled);
+      showToast('Ferienbanner-Einstellung gespeichert', 'e-toast-success');
+    } catch (e) {
+      const msg = e instanceof Error ? e.message : 'Fehler beim Speichern';
+      showToast(msg, 'e-toast-danger');
+    } finally {
+      setHolidayBannerBusy(false);
+    }
+  };
+
   const onSaveVideoSettings = async () => {
     setVideoBusy(true);
     try {
@@ -291,6 +318,34 @@ const Einstellungen: React.FC = () => {
           )}
         </div>
       </div>
+
+      {/* Dashboard Display Settings Card */}
+      <div className="e-card" style={{ marginTop: 20 }}>
+        <div className="e-card-header">
+          <div className="e-card-header-caption">
+            <div className="e-card-header-title">Dashboard-Anzeige</div>
+          </div>
+        </div>
+        <div className="e-card-content">
+          <div style={{ marginBottom: 16 }}>
+            <CheckBoxComponent
+              label="Ferienstatus-Banner auf Dashboard anzeigen"
+              checked={holidayBannerEnabled}
+              change={(e) => setHolidayBannerEnabled(e.checked || false)}
+            />
+            <div style={{ fontSize: '12px', color: '#666', marginTop: 4, marginLeft: 24 }}>
+              Zeigt eine Information an, ob ein Ferienplan für die aktive Periode importiert wurde.
+            </div>
+          </div>
+          <ButtonComponent
+            cssClass="e-primary"
+            onClick={onSaveHolidayBannerSetting}
+            disabled={holidayBannerBusy}
+          >
+            {holidayBannerBusy ? 'Speichere…' : 'Einstellung speichern'}
+          </ButtonComponent>
+        </div>
+      </div>
     </div>
   );
 
diff --git a/listener/listener.py b/listener/listener.py
index ce104b1..2cf05e7 100644
--- a/listener/listener.py
+++ b/listener/listener.py
@@ -26,6 +26,17 @@ engine = create_engine(DB_URL)
 Session = sessionmaker(bind=engine)
 
 
+def on_connect(client, userdata, flags, reasonCode, properties):
+    """Callback for when client connects or reconnects (API v2)."""
+    try:
+        # Subscribe on every (re)connect so we don't miss heartbeats after broker restarts
+        client.subscribe("infoscreen/discovery")
+        client.subscribe("infoscreen/+/heartbeat")
+        logging.info(f"MQTT connected (reasonCode: {reasonCode}); (re)subscribed to discovery and heartbeats")
+    except Exception as e:
+        logging.error(f"Subscribe failed on connect: {e}")
+
+
 def on_message(client, userdata, msg):
     topic = msg.topic
     logging.debug(f"Empfangene Nachricht auf Topic: {topic}")
@@ -87,14 +98,14 @@ def on_message(client, userdata, msg):
 
 
 def main():
-    mqtt_client = mqtt.Client(protocol=mqtt.MQTTv311, callback_api_version=2)
+    mqtt_client = mqtt.Client(protocol=mqtt.MQTTv311, callback_api_version=mqtt.CallbackAPIVersion.VERSION2)
     mqtt_client.on_message = on_message
+    mqtt_client.on_connect = on_connect
+    # Set an exponential reconnect delay to survive broker restarts
+    mqtt_client.reconnect_delay_set(min_delay=1, max_delay=60)
     mqtt_client.connect("mqtt", 1883)
-    mqtt_client.subscribe("infoscreen/discovery")
-    mqtt_client.subscribe("infoscreen/+/heartbeat")
 
-    logging.info(
-        "Listener gestartet und abonniert auf infoscreen/discovery und infoscreen/+/heartbeat")
+    logging.info("Listener gestartet; warte auf MQTT-Verbindung und Nachrichten")
     mqtt_client.loop_forever()
 
 
diff --git a/server/Dockerfile.dev b/server/Dockerfile.dev
index 0c7220f..04f63af 100644
--- a/server/Dockerfile.dev
+++ b/server/Dockerfile.dev
@@ -9,7 +9,7 @@ FROM python:3.13-slim
 # verbindet (gemäß devcontainer.json). Sie schaden aber nicht.
 ARG USER_ID=1000
 ARG GROUP_ID=1000
-RUN apt-get update && apt-get install -y --no-install-recommends locales curl git \
+RUN apt-get update && apt-get install -y --no-install-recommends locales curl git docker.io \
     && groupadd -g ${GROUP_ID} infoscreen_taa \
     && useradd -u ${USER_ID} -g ${GROUP_ID} --shell /bin/bash --create-home infoscreen_taa \
     && sed -i 's/# de_DE.UTF-8 UTF-8/de_DE.UTF-8 UTF-8/' /etc/locale.gen \
diff --git a/server/database.py b/server/database.py
index ae83f31..9d181ce 100644
--- a/server/database.py
+++ b/server/database.py
@@ -14,7 +14,9 @@ if not DB_URL:
     # Dev: DB-URL aus Einzelwerten bauen
     DB_USER = os.getenv("DB_USER", "infoscreen_admin")
     DB_PASSWORD = os.getenv("DB_PASSWORD", "KqtpM7wmNd&mFKs")
-    DB_HOST = os.getenv("DB_HOST", "db")  # IMMER 'db' als Host im Container!
+    # Dev container: use host.docker.internal or localhost if db container isn't on same network
+    # Docker Compose: use 'db' service name
+    DB_HOST = os.getenv("DB_HOST", "db")  # Default to db for Docker Compose
     DB_NAME = os.getenv("DB_NAME", "infoscreen_by_taa")
     DB_URL = f"mysql+pymysql://{DB_USER}:{DB_PASSWORD}@{DB_HOST}/{DB_NAME}"
 
diff --git a/server/init_defaults.py b/server/init_defaults.py
index b783135..ea5e92a 100644
--- a/server/init_defaults.py
+++ b/server/init_defaults.py
@@ -3,10 +3,22 @@ import os
 from dotenv import load_dotenv
 import bcrypt
 
-# .env laden
-load_dotenv()
+# .env laden (nur in Dev)
+if os.getenv("ENV", "development") == "development":
+    load_dotenv()
 
-DB_URL = f"mysql+pymysql://{os.getenv('DB_USER')}:{os.getenv('DB_PASSWORD')}@{os.getenv('DB_HOST')}:3306/{os.getenv('DB_NAME')}"
+# Use same logic as database.py: prefer DB_CONN, fallback to individual vars
+DB_URL = os.getenv("DB_CONN")
+if not DB_URL:
+    DB_USER = os.getenv("DB_USER", "infoscreen_admin")
+    DB_PASSWORD = os.getenv("DB_PASSWORD")
+    # In Docker Compose: DB_HOST will be 'db' from env
+    # In dev container: will be 'localhost' from .env
+    DB_HOST = os.getenv("DB_HOST", "db")  # Default to 'db' for Docker Compose
+    DB_NAME = os.getenv("DB_NAME", "infoscreen_by_taa")
+    DB_URL = f"mysql+pymysql://{DB_USER}:{DB_PASSWORD}@{DB_HOST}:3306/{DB_NAME}"
+
+print(f"init_defaults.py connecting to: {DB_URL.split('@')[1] if '@' in DB_URL else DB_URL}")
 engine = create_engine(DB_URL, isolation_level="AUTOCOMMIT")
 
 with engine.connect() as conn:
@@ -53,6 +65,7 @@ with engine.connect() as conn:
             ('video_autoplay', 'true', 'Autoplay (automatisches Abspielen) für Videos'),
             ('video_loop', 'true', 'Loop (Wiederholung) für Videos'),
             ('video_volume', '0.8', 'Standard Lautstärke für Videos (0.0 - 1.0)'),
+            ('holiday_banner_enabled', 'true', 'Ferienstatus-Banner auf Dashboard anzeigen'),
         ]
     
         for key, value, description in default_settings:
diff --git a/server/routes/groups.py b/server/routes/groups.py
index 4f31e15..a2b1732 100644
--- a/server/routes/groups.py
+++ b/server/routes/groups.py
@@ -8,7 +8,7 @@ from server.permissions import admin_or_higher, require_role
 from sqlalchemy import func
 import sys
 import os
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 
 sys.path.append('/workspace')
 
@@ -27,6 +27,14 @@ def get_grace_period():
     return int(os.environ.get("HEARTBEAT_GRACE_PERIOD_PROD", "170"))
 
 
+def _to_utc(dt: datetime) -> datetime:
+    if dt is None:
+        return None
+    if dt.tzinfo is None:
+        return dt.replace(tzinfo=timezone.utc)
+    return dt.astimezone(timezone.utc)
+
+
 def is_client_alive(last_alive, is_active):
     """Berechnet, ob ein Client als alive gilt."""
     if not last_alive or not is_active:
@@ -42,7 +50,10 @@ def is_client_alive(last_alive, is_active):
             return False
     else:
         last_alive_dt = last_alive
-    return datetime.utcnow() - last_alive_dt <= timedelta(seconds=grace_period)
+    # Vergleiche immer in UTC und mit tz-aware Datetimes
+    last_alive_utc = _to_utc(last_alive_dt)
+    now_utc = datetime.now(timezone.utc)
+    return (now_utc - last_alive_utc) <= timedelta(seconds=grace_period)
 
 
 @groups_bp.route("", methods=["POST"])
diff --git a/server/routes/system_settings.py b/server/routes/system_settings.py
index 646475d..64db2e4 100644
--- a/server/routes/system_settings.py
+++ b/server/routes/system_settings.py
@@ -202,3 +202,66 @@ def update_supplement_table_settings():
     finally:
         session.close()
 
+
+@system_settings_bp.route('/holiday-banner', methods=['GET'])
+def get_holiday_banner_setting():
+    """
+    Get holiday banner enabled status.
+    Public endpoint - dashboard needs this.
+    """
+    session = Session()
+    try:
+        setting = session.query(SystemSetting).filter_by(key='holiday_banner_enabled').first()
+        enabled = setting.value == 'true' if setting else True
+        
+        return jsonify({'enabled': enabled}), 200
+    except SQLAlchemyError as e:
+        return jsonify({'error': str(e)}), 500
+    finally:
+        session.close()
+
+
+@system_settings_bp.route('/holiday-banner', methods=['POST'])
+@admin_or_higher
+def update_holiday_banner_setting():
+    """
+    Update holiday banner enabled status.
+    Admin+ only.
+    
+    Request body:
+    {
+        "enabled": true/false
+    }
+    """
+    session = Session()
+    try:
+        data = request.get_json()
+        if not data:
+            return jsonify({'error': 'No data provided'}), 400
+        
+        enabled = data.get('enabled', True)
+        
+        # Update or create setting
+        setting = session.query(SystemSetting).filter_by(key='holiday_banner_enabled').first()
+        if setting:
+            setting.value = 'true' if enabled else 'false'
+        else:
+            setting = SystemSetting(
+                key='holiday_banner_enabled',
+                value='true' if enabled else 'false',
+                description='Ferienstatus-Banner auf Dashboard anzeigen'
+            )
+            session.add(setting)
+        
+        session.commit()
+        
+        return jsonify({
+            'enabled': enabled,
+            'message': 'Holiday banner setting updated successfully'
+        }), 200
+    except SQLAlchemyError as e:
+        session.rollback()
+        return jsonify({'error': str(e)}), 500
+    finally:
+        session.close()
+