Disneyland at Christmas time is beautiful! I think they decorate by mid-November and that would be the best time to avoid the heaviest crowds. They have a cute daytime Christmas parade, a Christmas version of World of Color, snow on Mainstreet and a Christmas version of the fireworks - I highly recommend all of them and I think they start those in mid November too but maybe someone else can chime in with that info. When we were this past Christmas time, they also had the Paint the Night parade. It was awesome! If they have it again, I would strongly recommend trying to see that. The Haunted Mansion is changed over to the Jack Skellington version in September I think. I love both versions of the Haunted Mansion, but I like the Christmas one more and seeing the outside of It's a Small World at night is really special. Christmas time at Disneyland is really really good. I haven't been to WDW during Christmas so can't compare, but we loved Disneyland at that time of the year.