Hi Lisa,
Walt Disney World has some amazing Christmas decorations. I always love to visit the parks and resorts when they are up.
Historically, the majority of the Christmas decorations will be up by mid-November. However, there are some special events that won't start until after Thanksgiving. The most important that comes to my mind is "Holidays Around the World" at Epcot. During this event, Epcot hosts themed storytellers in the various countries in World Showcase and the Candlelight Processional. The
Candlelight Processional is absolutely amazing. It should not be missed! It is one of the most moving things that I have ever experienced, inside or outside a Disney park. It's fabulous. In 2009, Holidays Around the World and the Candlelight Processional was offered from November 27th through December 30th. Disney has not yet released the dates for 2010.
My favorite time of the year is the first two weeks of December. Everything will be up and running for the holidays and the crowds are not "crazy" yet. I recommend you consider coming then.
Enjoy planning your vacation and write back if you have any more questions!