I think it may seem as though all decorations are up by mid-November, and, in fact, most of them are and they do begin to go up pretty much right after Halloween decor is removed, but it is done in phases. There are, for example, 700 Christmas trees that are put up around the entire DLR - in all the hotels, all the shops, some window displays, all the restaurants, DTD, both parks, everywhere. Some of the trees are small and some are gigantic. It would be hard to tell if ALL 700 trees and all the miles and miles of garland DLR uses were up by [fill in the blank date] last year. Keep in mind that if the 'official' holiday season starts on November 20, then that is when the Winter Castle-to-Main Street light show and the snow begin to happen. They won't do those things before November 20th. That is why November 21st was so packed last year (according to DIS-ers) - people piled in to see the light show and the Winter Castle lit up in its full glory on the day those events began - the offical start of the season.
But you will certainly see seasonal things go up throughout November, all building up to the magical time that is the DLR holiday season on 11/20.