Chances are pretty good that quite a few decorations should be up the week before Thanksgiving, but that Friday is the day they'll be in full Christmas mode. I know that last year they had the snow on the castle a week or so before they had the lights up on the castle. Haunted Mansion Holiday will be in full layover since it's up before October.
And it's a minor thing, but if you want to see the castle with the wreath on it, you must go before December 31. The wreath comes down on that day for the New Years banner.