diff --git a/client/src/pages/TripPlannerPage.test.tsx b/client/src/pages/TripPlannerPage.test.tsx
index 8f314c70..4babedc4 100644
--- a/client/src/pages/TripPlannerPage.test.tsx
+++ b/client/src/pages/TripPlannerPage.test.tsx
@@ -1160,7 +1160,7 @@ describe('TripPlannerPage', () => {
   });
 
   describe('FE-PAGE-PLANNER-041: handleSaveReservation edit path covers update reservation', () => {
-    it('preserves the reservation day_id on edit so it stays in the Plan (#1237)', async () => {
+    it('does not force a day_id on edit so the server keeps/derives it (#1237)', async () => {
       vi.useFakeTimers();
 
       seedTripStore({ id: 42 });
@@ -1196,12 +1196,10 @@ describe('TripPlannerPage', () => {
         });
       });
 
-      // The booking must keep its own day_id (not be nulled to the selected day).
-      expect(updateReservationSpy).toHaveBeenCalledWith(
-        expect.anything(),
-        1,
-        expect.objectContaining({ day_id: 7 }),
-      );
+      // The client must NOT send a day_id (no forcing to the selected day, no
+      // stale value) — the server keeps/derives it from the booking's date.
+      expect(updateReservationSpy).toHaveBeenCalled();
+      expect(updateReservationSpy.mock.calls[0][2]).not.toHaveProperty('day_id');
     });
   });
 
diff --git a/client/src/pages/tripPlanner/useTripPlanner.ts b/client/src/pages/tripPlanner/useTripPlanner.ts
index aef9c6a6..02bae54f 100644
--- a/client/src/pages/tripPlanner/useTripPlanner.ts
+++ b/client/src/pages/tripPlanner/useTripPlanner.ts
@@ -568,10 +568,12 @@ export function useTripPlanner() {
   const handleSaveReservation = async (data: Record<string, string | number | null> & { title: string }) => {
     try {
       if (editingReservation) {
-        // Keep the reservation on its own day. Forcing selectedDayId here dropped
-        // the booking out of the Plan when edited from the Book tab (no day
-        // selected -> selectedDayId null -> day_id nulled).
-        const r = await tripActions.updateReservation(tripId, editingReservation.id, { ...data, day_id: editingReservation.day_id ?? null })
+        // Don't force a day here. The old code pinned it to the (often empty)
+        // selected day, which dropped the booking out of the Plan; preserving the
+        // old day_id instead left it stale when the date changed. Omitting it lets
+        // the server derive the day from the booking's date, or keep the current
+        // one when there is no date.
+        const r = await tripActions.updateReservation(tripId, editingReservation.id, data)
         toast.success(t('trip.toast.reservationUpdated'))
         setShowReservationModal(false)
         setEditingReservation(null)
diff --git a/server/tests/integration/reservations.test.ts b/server/tests/integration/reservations.test.ts
index 9ff7fcc4..36c08f3a 100644
--- a/server/tests/integration/reservations.test.ts
+++ b/server/tests/integration/reservations.test.ts
@@ -200,6 +200,29 @@ describe('Update reservation', () => {
     expect(res.body.reservation.day_id).toBe(day2.id);
   });
 
+  it('RESV-004c — re-dating a booking moves it to the matching day (start + end) (#1237)', async () => {
+    const { user } = createUser(testDb);
+    const trip = createTrip(testDb, user.id);
+    const day1 = createDay(testDb, trip.id, { date: '2025-10-01' });
+    const day3 = createDay(testDb, trip.id, { date: '2025-10-03' });
+
+    // Booking sits on day 1 (start + end).
+    const created = await request(app)
+      .post(`/api/trips/${trip.id}/reservations`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Event', type: 'event', day_id: day1.id, reservation_time: '2025-10-01T09:00', reservation_end_time: '2025-10-01T10:00' });
+    const rid = created.body.reservation.id;
+
+    // Re-date to day 3 WITHOUT sending day_id (the modal omits it) — both ends follow.
+    const res = await request(app)
+      .put(`/api/trips/${trip.id}/reservations/${rid}`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Event', type: 'event', reservation_time: '2025-10-03T00:00', reservation_end_time: '2025-10-03T14:00' });
+    expect(res.status).toBe(200);
+    expect(res.body.reservation.day_id).toBe(day3.id);
+    expect(res.body.reservation.end_day_id).toBe(day3.id);
+  });
+
   it('RESV-004 — PUT on non-existent reservation returns 404', async () => {
     const { user } = createUser(testDb);
     const trip = createTrip(testDb, user.id);