Don't panic! The fact that you can see room available for that range of dates is good news, because it means one of the 300 peak-night rooms is still available. I'm guessing most people want more than those three nights, so once again, here's how you get the nights you want:
- Book a reservation for the three middle nights
- Write to the Sasquan Housing Bureau (link is on the hotels page) asking for the nights you actually want.
- The Housing Bureau will, when they get to you, pry loose additional nights from the Doubletree and extend your reservation
This process works. That's how I got the reservation I wanted (arriving Tuesday before the con, leaving the Tuesday after). But it doesn't happen instantaneously, and there are a lot of reservations backlogged while the Sasquan Housing Bureau works through them.
Obviously, this isn't the optimal solution. Sasquan would love to have more room nights on the shoulders, but while the Doubletree wouldn't give the nights to Sasquan up front, they will apparently continue to release dribs and drabs of rooms to the Housing Bureau. With no groups using up rooms either before or after Worldcon, it seems likely that the HB will be able to fill all of the demand, just more slowly than everyone would prefer they do.
Of course, you can bypass all of this entirely by staying at the Davenport (party hotel) or one of the two Red Lions (sleeping rooms only), where the shoulder-night restriction isn't affecting reservations. The key message here is that despite what people may have been saying, there are still rooms available at all four of the 2015 Worldcon's hotels as of this afternoon.
[Still not speaking officially for Sasquan. I'm only working from what I read on other sites and information being distributed to the committee, but that information internally is the same as what's being posted in public, so nobody is being kept in the dark.]