Can my customers book a hotel for the same night on my portal, or do they have to book in advance?

Yes, same night bookings are possible on your portal, but whether a particular hotel accepts one depends on the supplier the rate comes from and on how late in the day your customer is trying to book.

When a search is made for tonight, your portal asks the suppliers what is still open for that date and shows only what comes back. Hotels that have closed their availability for the day, or that have set a cut off time for arrivals, simply will not appear, so the list your customer sees late in the evening will be shorter than the list for next week. If the booking goes through, the confirmation and the voucher are produced in the usual way and the hotel is told to expect the guest.

The honest limits are worth explaining to your staff. Some suppliers stop accepting bookings for the current night at a fixed hour, and others pass the request to the hotel to accept, which is slower and does not suit a guest already on the way. A same night rate is very often non refundable, and if the guest arrives after the hotel's stated arrival time the room may not be held. None of that is something your portal can override, because the rules belong to the hotel and the supplier.

If same night business matters to you, ask your suppliers what their cut off times are and which of their hotels confirm instantly, and tell your staff to phone the hotel directly for a very late arrival so the front desk is expecting the guest.