Tink said it best.
Summers seem to be very manageable crowd wise. But then there's the heat. And also, there's a reason it's so busy in December...the holiday season brings amazing decorations, entertainment, and a very magical feeling to the parks. I went during the holiday season last year, but in November, and I also went this summer in August. Both fun trips but the holidays are just magical. So it depends on what you're looking for. Now, I haven't been in December. This is my first year going so close to Christmas so maybe I'll say differently after, but I've prepared myself for heavy crowds with reservations, tours, preferred seating, etc.