We were there last November and went to the first Christmas Party at the MK, on I think, November 9th. Decorations were just going up, you'd see a few more each day around the resorts and parks. Because of the parties, MK will be the first to be completely decorated. By mid Nov there should definitely be Christmas "in the air" around WDW properties.
The past few years we've been arriving the day after the F&W Festival is over. Sometimes this has coincided with the end of the Halloween celebration. We have seen our resort change from Halloween decorations to Christmas overnight on a couple of occasions. We normally leave for home on the Friday before Thanksgiving week. Many of the resorts are at least partially decorated, the Gingerbread House is "open" at the GF and the Osborne Lights are in full swing.