Be prepared for anything! I was there in December of 2001 and it was quite warm both days and nights. I was there last December and it was warm during the day and very, very cold at night. We saw Illuminations one night and were absolutely freezing. We had jeans and sweatshirts on - it wasn't enough.
I will be there this December and will pack for both warm and cold. I am also bring an infant (3 months) with me for the first time so I will be packing shorts, light clothes, and I will definately pack some warm clothes, too!
Good luck, but be prepared!
Kim