We like going late Oct. early Nov.
One year we were coming home from MNSSHP on halloween, we got back to the BCV about 12:30 in the morning and they were putting some of the decorations up at that time.
By the time we left at the end of the first week of Nov. the resort and quite a bit of the magic kingdom were decorated. It is not always that early but it will definatly be done by Thanksgiving.