As others have said, decorations start going up around property early in November and there are many special events going on leading up to and through Christmas. Keep in mind, however, that the week of Christmas through New Years is the single busiest time of the year at WDW. Parks regularly close due to capacity crowds during this week and it is wall to wall people. It's also the most expensive time to go in terms of rooms rates and those reservations are in high demand.
It's great - but you have to know what you're in for and be prepared. If you're considering going this year, you may already be out of luck getting a room on site.