MK will begin overnight as soon as the last Halloween party ends.
Google it and you can see time-lapse videos of how they do it, it's fascinating.
Other parks do it more slowly, later. Starting with their feature trees.
Each Deluxe resort is decorated in an order determined by lottery, timed so they finish the last one the weekend before Thanksgiving. They do each resort, one at a time, over a single night (another fun thing to Google to watch the time-lapse video). The other resorts are done in between in a smaller scale.
GF will begin doing their Gingerbread house in early November but it won't open till that weekend prior to Thanksgiving, or maybe even a few days before Thanksgiving.
We were there over Thanksgiving about 3 years ago.
Some of the parks were completely decorated for Christmas at the start of the week. By the end of the week, they were fully decked out in Christmas attire.
The resorts took a little longer as they were still building the Ginger Bread Houses. Don't miss them, they are amazing. we really enjoyed the Ginger Bread Houses at Wilderness, Yacht Club, and Grand Floridian
Sadly, most Gingerbread structures have slowly faded away, except for the ones at CR and GF. Last year they were the only 2 left, other than the Carousel at BC. GF is still the most elaborate by far.
When will the Christmas decorations come down? Arriving on Jan 10th.
All will be gone (literally overnight, same way they go up at the resorts) by the first weekend in January, except for the tree at MK. They will normally leave it up through the marathon weekend.
Any idea about the campsite decorations. We wanted to view them the Fri before Thanksgiving week. Now I am wondering if it will be too early.
That's solely up to the campers, not Disney.