We have been on 4 or 5 Thanksgiving week trips and every deluxe we have visited has been decorated by Thanksgiving. One year we were at WL the Monday before for breakfast at WCC and there was nothing and a few days later we went back again and it was.
About 11-12 years ago, there was a guy that used to frequent the forums...a huge WL fan. He insisted that all of the WDW resorts were always transformed by the morning of Nov 16th...that it would all magically happen overnight. Obviously, that practice has changed, but we got to experience it in 2012. We checked into Yacht Club on November 15th and everything looked normal. We got back late from Downtown Disney (hadn't changed to DS at that point) and saw several large trucks at YC and BC. Still, all looked normal at that point. We woke up on the 16th to head to breakfast at Cape May and YC and BC were in full on Christmas mode, just like that old member had promised. On one hand, I miss the predictability, but I guess it kind of adds to the magic when it's unexpected.