It used to be that the day after Thanksgiving saw all the decorations for Christmas up all over WDW. That is no longer the case. They take much longer now to put them up, so you have a good chance of seeing some the week before Thanksgiving.
As Christmas approaches there will be more and more set up. Some (EPCOT lighted archway and Christmas tree for examples) may not be in full effect until Christmas is very close.
It used to be that the Holiday story tellers of the WS were there the day after Thanksgiving until Christmas Eve, but even that isn't the case any more.
Things are still pretty enough, but nothing like it used to be.