They star as soon as they can get the halloween decorations down. Everything is done by the Saturday after Thanksgiving, with the exception of the MK tree. That does not go up until after parade taping the following weekend.
The parks gets started November 1st, and the castle lights are started even earlier. The first MVMCP at MK is November 9th, so all Christmas decorations at MK should be up by then. The resorts usually start the 2nd - 3rd week of November. I know in 2010, we stayed at CSR Thanksgiving week and when we arrived they were putting the decorations up and during the week we got to see them put up the tree in the lobby.
Last year, for the GF, decorations went up Monday night (of the week before Thanksgiving) November 14th. We arrived Monday morning ... (the Gingerbread House was already up) with very little other decorations and when we woke on Tuesday morning, everthing was up. We tranferred to the WL on Wednesday the 16th and it's decorations had already been completed. We did some resort hopping on Thursday the 17th and noted the Contemporary was still in progress (almost done) and the 3 Epcot resorts BWI, BC, YC) were completly done.