Last year they were Sunday, Tuesday, Thursday, Friday from Dec 1 to Dec 20.
I've attended every year since 1998 and I keep the buttons from each year on my cube wall at work, I was just looking at them today and day dreaming...
We always go either Sunday or Tuesday and it has never been real crowded. Fridays usually sellout. Although the official start time last year was 7:00pm, you could get in with your ticket as early at 4:00pm and get your wristband. It's a great idea to get there before the crowds.
http://wdwig.com/holiday.htm