Happy Solstice Day! (Although, technically, this year the exact Winter Solstice occurs at 1:08 AM EST tomorrow, most people generally recognize December 21 as Solstice Day.) Humans have been celebrating the Winter Solstice for at least 10,000 years. It is a time of renewal that, in the Christian era, has come to be associated with Christmas and the birth of Jesus. Christians tend to co-opt the season, however, even though its recognition predates their whole religion. I have to laugh at the signs and bumper stickers I see at this time of year that say "Jesus is the Reason for the Season" because some Christians don't realize that other people and beliefs exist.
Anyway, for a nice discussion of what the Winter Solstice is all about, go here.