We went last year 12-21. It was a wonderful time to go - the crowds were very manageable. However, the crowds began to arrive toward the end of the trip.
Weather wise, the day we arrived it was chilly, along with the next morning. I was in a t-shirt and jeans at RD @ MK on the 13th. The CMs were huddled in sweatshirts and everything. Coming from -10 to 50, I wasn't complaining. It was WARM as far as I was concerned!
The rest of the week it was in the high 70s/low 80s. Beautiful weather!