The official start for this year's holiday season is November 13th and that's when the holiday fireworks, parade, and tours will start. They have actually already started putting up some decorations. There is snow on the castle, people have reported seeing the Christmas tree back behind the tram service area fence, and various lands have garlands going up. Main Street (minus the tree) should be pretty well decorated by next weekend since they will be filming the Disneyland portion of the Disney Christmas special next weekend. Here's a link (http://mintcrocodile.blogspot.com/2009/10/clash-of-two-seasons.html) to some photos of what the mix of two holidays (Halloween and Christmas) looked like two days ago. If you scroll down through the photos, you'll see some details of garlands in New Orleans Square and other signs of holiday decor going up.
So, bottom line, if you are planning on being there during next year's Thanksgiving week, I think it's pretty safe to say you will have all the Christmas decorations in place.