Been 3 times in mid-December. The first time in 08 was picture perfect. Wore shorts and a t-shirt to the Christmas party. Didn't get too hot during the day (and I'm a total heat whimp).
2010 was a different story. Bitter cold. Used hand warmers, tons of layers plus blankets to get trough the party. Rained one morning ALL morning.
Last year in 2011 it was close to perfect agan. Highs near 80 (almost too hot by my whimpy standards) and rain just one night, a night we did the party. Got brutally and painfully soaked in a sudden downpour on BTMRR. But we still had an awesome time. And I got a new t-shirt and towel to bring home!

Although standing in the gift shop for half an hour of party time waiting for the rain to clear drove me NUTS!
We hate December weather so much we're going back again this year