They book it as part of a reservation that starts 7 days earlier, and then adjust to get the dates they really want--its referred to as walking.
Some, I have heard, even start more than that many days as it is so popular.
For example, say you want to go Dec 28th - January 2nd. Your 11 month opens on November 28. Instead, you call on November 22nd and book December 22nd to the 29th. You have now locked in your room ahead of others. On the 28th, you then call and adjust your dates to the ones you want.
Doing this gets you the room ahead of others and locks in that difficult night.