If you tried to book your hotel room for the 2015 Worldcon, Sasquan, today, and if you wanted to book a room at the Doubletree (the closest hotel, but not the one where the parties will be — that's the Davenport), you probably got a "no availability" message unless you only picked the range August 20-23. This does not really mean the hotel is sold out; it means that the hotel has not released to Sasquan enough "shoulder nights" in the initial room allocation. If you put that range August 20-23 into the web site, at least as of this evening, it still shows available rooms. Sasquan says on their hotel web site to book those core nights and write to the Housing Bureau (link on the hotel page) with your desired actual range of nights. The reports I've heard say that the Housing Bureau has been able to "dynamically add rooms" for each individual case.
As long as the core nights are still available, I expect that you will be able to get the shoulder nights. Once the core nights are gone, I expect that will be it; however, until that happens, don't let the lack of shoulder nights discourage you from making your booking and asking for the full range of nights.
(I am not speaking from insider information here. This is what is on the Sasquan web site and what has been posted in public elsewhere. I'm on the Sasquan committee as an area head, but my job isn't in the hotel/facilities division.)