When we were there for Halloween several years ago, we were shocked to enter the park on November 1 to find the Christmas up on Main Street USA. I think there were still some Halloween decorations left, as they probably cannot remove them all in one night, but not many, and I'm sure they didn't stay up for long.
Of course, the Nightmare Before Christmas theme will keep going at the Haunted Mansion through December, and that's sort of a Christmas/Halloween hybrid.