I think December is just where it is the worst - see my last post.
If you compare savings using points instead of cash rates, September is right behind December last I checked. And, again, September used to be one of those months people didn't go to WDW because kids were back in school. Well, you've got a lot of Disney adults and probably even plenty of families willing to take kids out of school for a week in September. And, with the points being the lowest and savings over rack rates, why not? Of course, still pretty darn hot and in the middle of hurricane season, so it isn't for me, but I understand the appeal.
Rack rates are fugazi.