I went to Disney in December of 2004. Some days it was freezing, and you would need to wear fleeces, and the very next day I would wear a t-shirt. It would still be cool, but the sun would be shinning on you. I would pack plenty of warm outfits and plenty of outfits where you could remove layers. It really just depends. Every day is different. The water parks are open in December...I never understood that. It's definatly not warm enough to be swimming, but people do it!!