diff --git a/server/src/services/reservationService.ts b/server/src/services/reservationService.ts
index 5385c6a9..4084220e 100644
--- a/server/src/services/reservationService.ts
+++ b/server/src/services/reservationService.ts
@@ -364,12 +364,19 @@ export function updateReservation(id: string | number, tripId: string | number,
   // otherwise derive from the (possibly updated) reservation_time so the
   // planner renders the booking on the correct day.
   let nextDayId: number | null;
-  if (day_id !== undefined) {
-    nextDayId = day_id || null;
-  } else if (reservation_time !== undefined && resolvedType !== 'hotel') {
+  if (day_id != null) {
+    // Explicit day from the client (e.g. moved on the planner).
+    nextDayId = day_id;
+  } else if (resolvedType !== 'hotel' && nextReservationTime) {
+    // No day set but we have a date — pin it to the matching day so the booking
+    // still shows in the Plan (covers bookings saved without a selected day, and
+    // the case where an earlier edit cleared day_id).
     nextDayId = resolveDayIdFromTime(tripId, nextReservationTime);
-  } else {
+  } else if (day_id === undefined) {
+    // Field absent and nothing to derive from — keep whatever it had.
     nextDayId = current.day_id ?? null;
+  } else {
+    nextDayId = null;
   }
 
   let nextEndDayId: number | null;
diff --git a/server/tests/integration/reservations.test.ts b/server/tests/integration/reservations.test.ts
index b86a28d5..9ff7fcc4 100644
--- a/server/tests/integration/reservations.test.ts
+++ b/server/tests/integration/reservations.test.ts
@@ -185,6 +185,21 @@ describe('Update reservation', () => {
     expect(res.body.reservation.confirmation_number).toBe('ABC123');
   });
 
+  it('RESV-004b — PUT with day_id null derives day_id from reservation_time so it stays in the Plan (#1237)', async () => {
+    const { user } = createUser(testDb);
+    const trip = createTrip(testDb, user.id);
+    createDay(testDb, trip.id, { date: '2025-09-01' });
+    const day2 = createDay(testDb, trip.id, { date: '2025-09-02' });
+    const resv = createReservation(testDb, trip.id, { title: 'Event', type: 'event' });
+
+    const res = await request(app)
+      .put(`/api/trips/${trip.id}/reservations/${resv.id}`)
+      .set('Cookie', authCookie(user.id))
+      .send({ title: 'Event', type: 'event', day_id: null, reservation_time: '2025-09-02' });
+    expect(res.status).toBe(200);
+    expect(res.body.reservation.day_id).toBe(day2.id);
+  });
+
   it('RESV-004 — PUT on non-existent reservation returns 404', async () => {
     const { user } = createUser(testDb);
     const trip = createTrip(testDb, user.id);