There is no exact set date for starting/completing certain resorts decorations.
They start right after the halloween decorations are down and continue until they are ALL up the Saturday after Thanksgiving, with the exception of the MK tree, which is put up after the parade taping the following weekend.
Many of the trees go up overnight, while the more intricate displays like the Gingerbread house at GF and the carousel at BW take a lot longer.
Everything stays up until a day or two after NYE, with the exception of the last few years, when they were hit by a cold snap and had to delay it a few days. Everything comes down very, very quickly.