That really depends on which resort you want to stay at. Saratoga Springs is always the last resort to sell out (because of its size) and I think this year, you could get a Christmas reservation right up to the 5 month mark.
WIlderness is the quickest to sell out and should be booked between 10 and 11 months. Bay Lake Tower is next, followed by Beach Club then Boardwalk. I recommend 8 to 10 months in advance. Once the 7 month window opens, it's open season.
Of course, these opinions may change do many factors. To be sure you get a room, call at the 11 month mark.
New Year's Eve is a different story and should be booked at the 11 month mark at the popular/smalle resorts.
David