Historically, it's usually held the first Saturday and Sunday of December. It's also extremely crowded on those days, but if that's why you're there, it is very worth it.
It has consistently been the first weekend (Sat./Sun) in December, so everybody is guessing that it will be Dec. 5th and 6th this year. They have 2 shows each day at 5:30 and 8:00. We went last year and we loved it!.