Transformation of MK is usually overnight after the final Halloween (Boo Bash) event. (Oct. 31 this year) Other parks are slowly transformed, generally complete by Thanksgiving week. Decorations start coming down after NYE but are usually up through Marathon weekend (Jan 9/10 this year). Resort decorations are taken down after the 1st on a staggered schedule.
Of course, this is all pre-pandemic information.