diff --git a/client/src/pages/TripPlannerPage.test.tsx b/client/src/pages/TripPlannerPage.test.tsx
index 9512c3ec..8f314c70 100644
--- a/client/src/pages/TripPlannerPage.test.tsx
+++ b/client/src/pages/TripPlannerPage.test.tsx
@@ -1160,10 +1160,13 @@ describe('TripPlannerPage', () => {
   });
 
   describe('FE-PAGE-PLANNER-041: handleSaveReservation edit path covers update reservation', () => {
-    it('calls onEdit then onSave on ReservationModal to exercise the edit-reservation handler', async () => {
+    it('preserves the reservation day_id on edit so it stays in the Plan (#1237)', async () => {
       vi.useFakeTimers();
 
       seedTripStore({ id: 42 });
+      // Capture the update payload — tripActions is a snapshot of the store at mount.
+      const updateReservationSpy = vi.fn().mockResolvedValue({ id: 1, day_id: 7 });
+      seedStore(useTripStore, { updateReservation: updateReservationSpy } as any);
 
       renderPlannerPage(42);
 
@@ -1179,20 +1182,26 @@ describe('TripPlannerPage', () => {
         expect(screen.getByTestId('reservations-panel')).toBeInTheDocument();
       });
 
-      // Set editingReservation via captured onEdit prop (inline lambda in JSX)
-      const fakeReservation = { id: 1, trip_id: 42, name: 'Test', type: 'restaurant', status: 'confirmed' };
+      // Edit a reservation that lives on day 7 (no day is selected — Book tab).
+      const fakeReservation = { id: 1, trip_id: 42, name: 'Test', type: 'other', status: 'confirmed', day_id: 7 };
       await act(async () => {
         capturedReservationsPanelProps.current.onEdit?.(fakeReservation);
       });
 
-      // Call onSave — now takes edit path (editingReservation is set)
       await act(async () => {
         await capturedReservationModalProps.current.onSave?.({
           name: 'Updated Booking',
-          type: 'restaurant',
+          type: 'tour',
           status: 'confirmed',
         });
       });
+
+      // 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 }),
+      );
     });
   });
 
diff --git a/client/src/pages/tripPlanner/useTripPlanner.ts b/client/src/pages/tripPlanner/useTripPlanner.ts
index 03d6d094..aef9c6a6 100644
--- a/client/src/pages/tripPlanner/useTripPlanner.ts
+++ b/client/src/pages/tripPlanner/useTripPlanner.ts
@@ -568,7 +568,10 @@ export function useTripPlanner() {
   const handleSaveReservation = async (data: Record<string, string | number | null> & { title: string }) => {
     try {
       if (editingReservation) {
-        const r = await tripActions.updateReservation(tripId, editingReservation.id, { ...data, day_id: selectedDayId || null })
+        // 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 })
         toast.success(t('trip.toast.reservationUpdated'))
         setShowReservationModal(false)
         setEditingReservation(null)