Hi, Andrew! I love a guy that plans ahead!
At the moment, advanced dining reservations can be made up to 180 days in advance. If you are a Walt Disney World resort guest, you can make your reservations for up to 10 days on that first day that you call, which basically means that you can call at 180 days in advance of your date of arrival, and make reservations for up to 10 days of your vacation.
I guess the great places to eat for the holidays really depends on what you are hoping to enjoy. Many of the restaurants will have some sort of holiday offering on the menu, and in years past, Whispering Canyon Cafe has offered a family style holiday meal that my gang has enjoyed. Another family style meal is at Liberty Tree Tavern at the Magic Kingdom for dinner. It just reminds us when we are traveling away from extended family of the meals we might be having at home.
Other fabulous restaurants we've enjoyed during the holidays include Artist Point at Wilderness Lodge, 1900 Park Fare at the Grand Floridian, and Narcoossee's at the Grand Floridian. If you are looking for a different flavor to your holiday celebration, I highly recommend Jiko at Animal Kingdom Lodge or Sanaa at Animal Kingdom Lodge Kidani Village.
Have a great time planning, and an even better time once you get there!