If you have always gone in the summer, I would say go in December. If you could go before Christmas it would be better, though. December is so much less crowded up until around December 22-3, usually. It has such a wonderful feel in the air. The week between Christmas and New Years has long hours, too I believe, if that is when you are definitely planning on going. If you have never experienced it decorated for Christmas I say do it. You will also get to go on Haunted Mansion Holiday and IASW Holiday. Even if you have to go the week of Christmas, although it will be very crowded, it is so wonderful.