Yes, Christmas is up and parties use to start the first week in November at magic kingdom. But then it takes awhile to decorate the resorts, as Dave said, on a lottery system. We have stayed at WL for thanksgiving, started with 2 weeks before for years, now stay the week after .
anyway, the tree has been up as early as the Monday, the week before thanksgiving and one year as late as the Tuesday of thanksgiving week. I would look at nov 15 or 16th and try to stay till nov. 22nd. For lower crowds and some decorations