We were there last Dec. 19-23. The weather was so warm during the day (70's and 80's.) At night, it was down in the 50's. Very nice. I wore a sleevless shirt during the day, and brought along a light jacket or sweater for night in the parks.
I have heard it is hit or miss in December. It can be very cold sometimes according to some of the CM's we talked to during our Dec. trip.