I just wanted to add to Ethansmom's comments:
Dec 20 -31 is Holiday Season. If your stay begins at any point within that period, your whole stay will be charged at the most expensive rate Disney has.
If your stay begins on or before Dec. 19 OR on or after Jan. 1, you will be in Value Season and your entire stay will be billed at the cheapest rate that Disney offers.
Knowing when the seasons begin and end can make a BIG difference in what you end up paying for your room!