We went the week before Thanksgiving in 2015. We got to see the very end of Food & Wine Festival and the beginning of the decorations of Epcot, the Gingerbread house at GF and carousel at BC, the Christmas party at MK. Overall we loved it...the weather was nice and we got plenty of Christmas Spirit.
This year, however, we are going the last week of November, checking in the Sat after Thanksgiving. As I was standing in Epcot on our second to last day in 2015, I saw the beginnings of the Candlelight Processional Set and the stands and decided right then and there that I would be back, with a gluhwein in hand and watching that show.
So, if you think this may be your one and only Christmastime there, you may want to consider the week after Thanksgiving (we check out on December 1) so you get the full Epcot experience, but if you are cool with everything except the Epcot experience then I suggest the week before Thanksgiving because you get most of the Christmas experience. Either way the crowds are on the lighter side. I think the weather is decent either way, on average, for Florida...warm but not crippling hot.
Have fun planning!!!