Most rooms (apart from special requests and multi-room bookings which need to be near to each other) are allocated on the actual day you arrive. This is to ensure that as many people as possible can be sent directly to a currently vacant room when they arrive (even at 9am) rather than having to wait for a pre-allocated room number to be vacated and cleaned. Guests who have done Online Checkin get allocated from the currently available pool of rooms a few hours before they are due to arrive (and a welcome pack is prepared for them). Walk-ups get allocated from the pool of 'Ready Rooms' at the time you get to the front desk. That's one reason I recommend online checkin by the way; if you don't like the room that was assigned a few hours ago you can ask them to check again from the current pool, effectively giving you a 'double dip' advantage if you need it. There is an option in the Online Checkin form to prioritise Requests over getting a room immediately, which means they can look for other rooms that might be due to become available later in the day, but at the expense of you having to wait a while for your room before you can get in. If you are especially worried, faxing 3-5 days out is a good idea, explaining that you are going to be checking in very late and would really like building 16 if possible, and saying that you are worried there might be no ideal rooms left there when you arrive.
Thank you! I will send a fax and cross my fingers!!