diff --git a/apps/app/src/composables/timetable/useActiveDay.ts b/apps/app/src/composables/timetable/useActiveDay.ts
new file mode 100644
index 00000000..d5995fd4
--- /dev/null
+++ b/apps/app/src/composables/timetable/useActiveDay.ts
@@ -0,0 +1,71 @@
+import { computed, watch } from 'vue'
+import type { Ref } from 'vue'
+
+/**
+ * `?day` query param ↔ active sub-event id binding for the timetable page.
+ *
+ * The URL is the source of truth. The store does NOT hold this value.
+ *
+ * Behaviour (RFC v0.2 §6.2 / Session 4 follow-up Step 5):
+ *  - If `?day=X` is in `validIds`         → activeDayId = X.
+ *  - If `?day=X` is missing or invalid    → activeDayId = first valid id,
+ *                                           and the URL is silently rewritten
+ *                                           via `replace({day: firstValidId})`.
+ *  - If `validIds` is empty (event has    → activeDayId = null.
+ *    no sub-events / data still loading)
+ *
+ * Cross-org sub-event IDs are blocked transparently: `OrganisationScope`
+ * on the backend never returns them, so they fail `validIds.includes(...)`
+ * and fall back to the first valid id from the user's own organisation.
+ */
+export interface UseActiveDayDeps {
+
+  /** Reactive read of `route.query.day`. Must coerce array → string outside. */
+  queryDay: Ref<string | null>
+
+  /** Reactive list of sub-event IDs the user is allowed to see for this event. */
+  validIds: Ref<string[]>
+
+  /** `router.replace` wrapper that updates ONLY the `day` query param. */
+  replace: (dayId: string) => void
+}
+
+export interface UseActiveDayReturn {
+  activeDayId: Ref<string | null>
+  setActiveDay: (id: string | null) => void
+}
+
+export function useActiveDay(deps: UseActiveDayDeps): UseActiveDayReturn {
+  const activeDayId = computed<string | null>(() => {
+    const ids = deps.validIds.value
+    if (ids.length === 0)
+      return null
+    const q = deps.queryDay.value
+    if (q && ids.includes(q))
+      return q
+
+    return ids[0]
+  })
+
+  // Single corrective watcher — quietly rewrites the URL when the query param
+  // is missing or invalid. immediate:true so a mount with `?day=null` (or
+  // an invalid value) corrects the URL on first paint instead of waiting
+  // for the next user interaction. flush:'post' so it runs after Vue settles
+  // and after validIds has been recomputed from a fresh fetch.
+  watch([() => deps.queryDay.value, () => deps.validIds.value], ([q, ids]) => {
+    if (ids.length === 0)
+      return
+    if (q === null || !ids.includes(q))
+      deps.replace(ids[0])
+  }, { flush: 'post', immediate: true })
+
+  function setActiveDay(id: string | null): void {
+    if (id === null)
+      return
+    if (id === deps.queryDay.value)
+      return
+    deps.replace(id)
+  }
+
+  return { activeDayId, setActiveDay }
+}
diff --git a/apps/app/src/pages/events/[id]/timetable/index.vue b/apps/app/src/pages/events/[id]/timetable/index.vue
index b0772b8f..6636a9ba 100644
--- a/apps/app/src/pages/events/[id]/timetable/index.vue
+++ b/apps/app/src/pages/events/[id]/timetable/index.vue
@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, onMounted, ref, watch } from 'vue'
+import { computed, ref, watch } from 'vue'
 import EventTabsNav from '@/components/events/EventTabsNav.vue'
 import AddPerformanceDialog from '@/components/timetable/AddPerformanceDialog.vue'
 import EmptyDayState from '@/components/timetable/EmptyDayState.vue'
@@ -14,6 +14,7 @@ import Wachtrij from '@/components/timetable/Wachtrij.vue'
 import { useEventChildren, useEventDetail } from '@/composables/api/useEvents'
 import { useTimetable } from '@/composables/api/useTimetable'
 import { useTimetableMutations } from '@/composables/api/useTimetableMutations'
+import { useActiveDay } from '@/composables/timetable/useActiveDay'
 import { useDragOrClick } from '@/composables/timetable/useDragOrClick'
 import { useTimetableKeyboard } from '@/composables/timetable/useTimetableKeyboard'
 import { generateIdempotencyKey } from '@/lib/idempotencyKey'
@@ -59,22 +60,31 @@ const dayOptions = computed(() => {
   return (subEvents.value ?? []).map(c => ({ id: c.id, name: c.name, start_date: null }))
 })
 
-const dayIdRef = computed<string | null>(() => store.activeDayId)
+// `?day` is the source of truth — Pinia derives from it, never owns it.
+// Behaviour fully delegated to useActiveDay (RFC §6.2 / Session 4 follow-up
+// Step 5). Cross-org IDs are blocked transparently: OrganisationScope never
+// returns them, so they fail validIds.includes(…) and fall back.
 
-watch(() => route.query.day, raw => {
-  const value = typeof raw === 'string' ? raw : null
-  if (value && value !== store.activeDayId)
-    store.setActiveDay(value)
-}, { immediate: true })
+const validSubEventIds = computed(() => dayOptions.value.map(o => o.id))
 
-watch(dayOptions, opts => {
-  if (!store.activeDayId && opts.length > 0)
-    store.setActiveDay(opts[0].id)
+const queryDay = computed<string | null>(() => {
+  const v = route.query.day
+
+  return typeof v === 'string' && v.length > 0 ? v : null
 })
 
-watch(() => store.activeDayId, id => {
-  if (id && id !== route.query.day)
-    router.replace({ query: { ...route.query, day: id } })
+const { activeDayId: dayIdRef, setActiveDay } = useActiveDay({
+  queryDay,
+  validIds: validSubEventIds,
+  replace: id => {
+    void router.replace({ query: { ...route.query, day: id } })
+  },
+})
+
+// Side effects on day-change: clear drag snapshot + deselect.
+watch(dayIdRef, () => {
+  store.endDrag()
+  store.selectPerformance(null)
 })
 
 const tt = useTimetable(orgId, eventId, dayIdRef)
@@ -377,11 +387,8 @@ const { announce } = useTimetableKeyboard({
   },
 })
 
-onMounted(() => {
-  // Fire watch immediately to bind ?day → store.
-  if (typeof route.query.day === 'string')
-    store.setActiveDay(route.query.day)
-})
+// `?day` source-of-truth is wired via the dayIdRef computed + the
+// corrective watcher above; no onMounted bootstrap needed.
 </script>
 
 <template>
@@ -390,9 +397,9 @@ onMounted(() => {
       <div class="tt-page">
         <div class="tt-page__toolbar">
           <VTabs
-            :model-value="store.activeDayId"
+            :model-value="dayIdRef"
             density="compact"
-            @update:model-value="v => store.setActiveDay(typeof v === 'string' ? v : null)"
+            @update:model-value="v => setActiveDay(typeof v === 'string' ? v : null)"
           >
             <VTab
               v-for="d in dayOptions"
diff --git a/apps/app/src/stores/useTimetableStore.ts b/apps/app/src/stores/useTimetableStore.ts
index e5618a6d..4995e7f6 100644
--- a/apps/app/src/stores/useTimetableStore.ts
+++ b/apps/app/src/stores/useTimetableStore.ts
@@ -15,14 +15,17 @@ import type {
  * cache via useTimetable.ts — this store carries only the bits that
  * multiple components on the canvas need to share:
  *
- *   - Active sub-event id (synced ↔ ?day query)
  *   - Selected performance id (drives popover anchor + keyboard focus)
  *   - In-flight drag state + origin snapshot for rollback (RFC D7)
  *   - Status filter (chips above wachtrij + canvas dimming)
  *   - Free-text search (wachtrij filter)
+ *
+ * Active sub-event ("day") is intentionally NOT held here — the URL
+ * `?day` query param is the source of truth, derived in the page entry
+ * via a computed against the validated sub-event list. Keeping it out
+ * of Pinia avoids the desync class of bug between URL and store.
  */
 export const useTimetableStore = defineStore('timetable', () => {
-  const activeDayId = ref<string | null>(null)
   const selectedPerformanceId = ref<string | null>(null)
 
   // Drag state — set by usePointerDrag handlers, consumed by mutation
@@ -51,10 +54,6 @@ export const useTimetableStore = defineStore('timetable', () => {
 
   const searchQuery = ref('')
 
-  function setActiveDay(id: string | null): void {
-    activeDayId.value = id
-  }
-
   function selectPerformance(id: string | null): void {
     selectedPerformanceId.value = id
   }
@@ -97,7 +96,6 @@ export const useTimetableStore = defineStore('timetable', () => {
   const isDragging = computed(() => dragPerformanceId.value !== null)
 
   return {
-    activeDayId,
     selectedPerformanceId,
     dragPerformanceId,
     dragOriginSnapshot,
@@ -105,7 +103,6 @@ export const useTimetableStore = defineStore('timetable', () => {
     statusFilter,
     searchQuery,
     isDragging,
-    setActiveDay,
     selectPerformance,
     startDrag,
     updateDragGhost,
diff --git a/apps/app/tests/unit/pages/timetableDaySync.test.ts b/apps/app/tests/unit/pages/timetableDaySync.test.ts
new file mode 100644
index 00000000..2563e2f7
--- /dev/null
+++ b/apps/app/tests/unit/pages/timetableDaySync.test.ts
@@ -0,0 +1,165 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { defineComponent, h, nextTick, ref } from 'vue'
+import { mount } from '@vue/test-utils'
+import { useActiveDay } from '@/composables/timetable/useActiveDay'
+
+/**
+ * The `?day` source-of-truth contract (Session 4 follow-up Step 5).
+ *
+ * useActiveDay is mounted inside a host component so the watcher actually
+ * runs (composables that use watch() need an active component instance).
+ */
+
+interface HostExposed {
+  activeDayId: () => string | null
+  setActiveDay: (id: string | null) => void
+  setQueryDay: (id: string | null) => void
+  setValidIds: (ids: string[]) => void
+  replaceMock: ReturnType<typeof vi.fn>
+}
+
+function mountSync(initialQuery: string | null, initialValidIds: string[]) {
+  const queryDay = ref<string | null>(initialQuery)
+  const validIds = ref<string[]>(initialValidIds)
+
+  const replaceMock = vi.fn((id: string) => {
+    queryDay.value = id
+  })
+
+  const host = defineComponent({
+    setup(_, { expose }) {
+      const { activeDayId, setActiveDay } = useActiveDay({
+        queryDay,
+        validIds,
+        replace: replaceMock,
+      })
+
+      expose({
+        activeDayId: () => activeDayId.value,
+        setActiveDay,
+        setQueryDay: (v: string | null) => { queryDay.value = v },
+        setValidIds: (v: string[]) => { validIds.value = v },
+        replaceMock,
+      })
+
+      return () => h('div')
+    },
+  })
+
+  const wrapper = mount(host)
+
+  return { wrapper, vm: wrapper.vm as unknown as HostExposed, replaceMock }
+}
+
+describe('useActiveDay — ?day source-of-truth', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+  })
+
+  it('valid ?day=X returns X without rewriting the URL', async () => {
+    const { vm, replaceMock } = mountSync('day_2', ['day_1', 'day_2', 'day_3'])
+
+    await nextTick()
+
+    expect(vm.activeDayId()).toBe('day_2')
+    expect(replaceMock).not.toHaveBeenCalled()
+  })
+
+  it('missing ?day → fallback to first valid id + URL replaced once', async () => {
+    const { vm, replaceMock } = mountSync(null, ['day_1', 'day_2', 'day_3'])
+
+    await nextTick()
+    await nextTick()
+
+    expect(vm.activeDayId()).toBe('day_1')
+    expect(replaceMock).toHaveBeenCalledTimes(1)
+    expect(replaceMock).toHaveBeenCalledWith('day_1')
+  })
+
+  it('invalid ?day=DOES_NOT_EXIST → fallback + URL corrected', async () => {
+    const { vm, replaceMock } = mountSync('day_bogus', ['day_1', 'day_2'])
+
+    await nextTick()
+    await nextTick()
+
+    expect(vm.activeDayId()).toBe('day_1')
+    expect(replaceMock).toHaveBeenCalledWith('day_1')
+  })
+
+  it('cross-org ?day (id absent from validIds) → fallback to first valid', async () => {
+    // Backend OrganisationScope keeps the cross-org sub-event out of the
+    // returned list; useActiveDay treats it identically to "doesn't exist".
+    const { vm, replaceMock } = mountSync('day_other_org', ['day_a'])
+
+    await nextTick()
+    await nextTick()
+
+    expect(vm.activeDayId()).toBe('day_a')
+    expect(replaceMock).toHaveBeenCalledWith('day_a')
+  })
+
+  it('empty validIds → activeDayId is null and URL is not touched', async () => {
+    const { vm, replaceMock } = mountSync('day_1', [])
+
+    await nextTick()
+    await nextTick()
+
+    expect(vm.activeDayId()).toBeNull()
+    expect(replaceMock).not.toHaveBeenCalled()
+  })
+
+  it('setActiveDay(id) calls replace with the new id', async () => {
+    const { vm, replaceMock } = mountSync('day_1', ['day_1', 'day_2'])
+
+    await nextTick()
+
+    vm.setActiveDay('day_2')
+    await nextTick()
+
+    expect(replaceMock).toHaveBeenLastCalledWith('day_2')
+    expect(vm.activeDayId()).toBe('day_2')
+  })
+
+  it('setActiveDay(null) is a no-op', async () => {
+    const { vm, replaceMock } = mountSync('day_1', ['day_1', 'day_2'])
+
+    await nextTick()
+    replaceMock.mockClear()
+
+    vm.setActiveDay(null)
+    await nextTick()
+
+    expect(replaceMock).not.toHaveBeenCalled()
+  })
+
+  it('external URL change (browser back) propagates to activeDayId', async () => {
+    const { vm } = mountSync('day_1', ['day_1', 'day_2'])
+
+    await nextTick()
+    expect(vm.activeDayId()).toBe('day_1')
+
+    vm.setQueryDay('day_2')
+    await nextTick()
+
+    expect(vm.activeDayId()).toBe('day_2')
+  })
+
+  it('validIds populated AFTER mount triggers fallback if ?day was missing', async () => {
+    const { vm, replaceMock } = mountSync(null, [])
+
+    await nextTick()
+    expect(vm.activeDayId()).toBeNull()
+    expect(replaceMock).not.toHaveBeenCalled()
+
+    vm.setValidIds(['day_1', 'day_2'])
+    await nextTick()
+    await nextTick()
+
+    expect(vm.activeDayId()).toBe('day_1')
+    expect(replaceMock).toHaveBeenCalledWith('day_1')
+  })
+})
diff --git a/apps/app/tests/unit/stores/useTimetableStore.test.ts b/apps/app/tests/unit/stores/useTimetableStore.test.ts
index 06f22ced..20f2fdb7 100644
--- a/apps/app/tests/unit/stores/useTimetableStore.test.ts
+++ b/apps/app/tests/unit/stores/useTimetableStore.test.ts
@@ -41,11 +41,14 @@ describe('useTimetableStore', () => {
     expect(store.isStatusVisible(ArtistEngagementStatus.CONFIRMED)).toBe(true)
   })
 
-  it('setActiveDay updates and selectPerformance maps to id', () => {
+  it('selectPerformance maps to id and clears on null', () => {
+    // activeDayId / setActiveDay intentionally REMOVED from the store
+    // (Step 5: ?day URL is the source of truth, page derives via computed).
     const store = useTimetableStore()
 
-    store.setActiveDay('day_1')
-    expect(store.activeDayId).toBe('day_1')
+    expect((store as unknown as { activeDayId?: string }).activeDayId).toBeUndefined()
+    expect((store as unknown as { setActiveDay?: unknown }).setActiveDay).toBeUndefined()
+
     store.selectPerformance('p1')
     expect(store.selectedPerformanceId).toBe('p1')
     store.selectPerformance(null)