Pack all your clothes & wear layers! LOL
Really, we went 12/1 - 12/10 last year. Daytime highs ranged from 50's to 80's. Lows were high 30's to 50's. And Florida winters are a DAMP COLD! We are from New England and we were cold!!! Some days I had 3-4 layers on. Tshirt, with light thermal shirt or sweatshirt with fleece pull over on top & at night added a fleece sleeveless vest. One night, I wore gloves. But by the end of our trip, we were in shorts & tshirts.
Pack it all. I kept hearing this over & over, and I was glad we brought such a wide range. We used it all. Even went swimming at night - pools are heated. Just run back to the room. LOL (From New England - we HAD to go swimming - it's Florida!!!)
Carry the layers with you in backpack. Temps change very quickly. Stash in a locker if you wish.
December is great!!!!! No matter what the weather. Decorations, music, no crowds..... Have a ball!