A Hubble Advent Calendar?
Cool.
So Boston.com has a Hubble Advent Calendar.
Damn, I wish I had thought of that.
STSCI, as before, has Hubble Seasonal Greeting Cards, free, for you to print out old style, or to send electronically.
and, for the classically minded, there is the BBC's A Bach Christmas Advent Calendar
How about a Chandra Icelandic Winter-Solstice Elves Advent Calendar?
Don't forget the planetary advent calendar:
http://planetary.org/blog/article/00002238/