We get to WDW the Sunday after Thanksgiving and it is totally decorated then. Not sure how much earlier they are up before that.
If you are going, make sure you take time to resort hop and see all of their decorations. We normally either walk from MGM or EPCOT and head over to the Dolphin, Swan, Boardwalk, Beach Club and Grand Floridian. This year we are hoping to find time to head to Wilderness Lodge.