What day is the reservation for1a.m.

Booking at this time should be counted as the day before.

One day (that is, one night) in a hotel usually refers to noon on the day of check-in to noon the next day. So 1 arrived at the hotel in the morning, and it still belonged to the room of the previous day. The default stay time is from noon the day before to noon the day before.

Therefore, if you stay in the hotel at 1 am, you should make a reservation the day before. However, the specific regulations of each hotel may be different, so when booking, it is best to check the hotel's occupancy policy in detail and confirm with the hotel in advance.