If you were there in Dec. 2010 from about the 5th - 15th (approx. can't rem. exactly now) it was freezing time. We were there and have pics to prove how cold it was. It was in the high 20's several nights and 40-50's with wind during the day. A lot of the plants froze ( had "jack frost visited us last night" signs) and the poinsettia "trees" were mostly covered, and several other plant beds were also under wraps. There was ICE on the MK dock in the morning

It does vary from year to year and that was a drastic round of lows for Dec. We're going same time this year and hope for some

We needed gloves, hats, heavy coats and scarves and we were still miserable for a lot of the time - viewing the Osbourne Lights and trying to take videos wasn't much fun either.
So OP, to be safe, take all kinds of clothes because you never know

Check PP's post and it will agree with mine!