We went the first week in December in 2001 and the weather was beautiful, we wore shorts during the day, and light pants and a jacket a night.
In 2002 I went the 14th of December to the 19th and the first few days of the trip were very very cold. My son and I actually wore our winter coats in the evening. During the day we wore long pants and wore a jacket. The day we left was very nice, warm enough to wear shorts.
The best thing to do is to bring a variety of clothes so that you can layer.
I am going the beginning of December this year, I will definately be following the 15 day forecast before going.