We had to start playing Christmas music the Friday after Thanksgiving. We play it very quietly, however, mostly because we can't stand it blaring all day long. We also avoid novelty garbage like the Muppets or Alvin and the Chipmunks. Classical holiday music is always a good bet, because it's not screaming "Christmas!" at you, and for some reason traditional crooners (Sinatra, Bennett, Martin, etc.) always make the customers smile.
Part of the reason that stores get their Christmas stuff out early is simply because there's so much to do in preparation for the weekend after Christmas that it takes days to get everything done. It's not just sticking up decorations, it usually involves changing every single display in a store, usually setting up special sale displays, and in our case many of our standard table displays are moved all over the place.
-Wm.