Parks are the first to get decorations… in the past, it’s usually a quick turnover from Halloween to Christmas the first week of November, especially in MK. The others are close behind, but usually finish by mid-November. The resorts slowly get decorated… I have always heard it was a lottery system as far as when each one gets done. But from what it has been in the past, resorts start around mid-November and anything with a more elaborate tree and decorations (such as the gingerbread houses) typically get done shortly before Thanksgiving. It’s definitely the most magical time to visit, IMO. Enjoy it!!!
ETA: resorts were decorated a bit earlier last year, so it’s definitely possible they will do that again this year! Until we start getting some live reports from people there, we won’t really know.