I am shocked by the number of people who don't realize that outside the name "Christmas" the holiday has NOTHING to do with Christianity!
The tree and other symbols pre-date Christianity by thousands of years, as does the date of its celebration. No one is even sure that the Jesus as described in the Bible was even a real person, much less when his friggin' birthday might have been. (There was probably a real person at the root of the story, but the details are all conjecture - similar to Robin Hood or King Arthur. Also, NONE of the Biblical accounts of his life were written until a hundred years or so after he was supposed to have lived.) If you do take the Bible literally, all indications are that he was probably born in the late summer/early fall.
The Romans decided to throw a Christian name on the Saturnalia and the celebration of the birth of Sol Invictus, to more easily convert the Pagan holdouts to their newly-adopted religion. In the U.S. and Britain the Puritans (you know, like those guys who gave us Thanksgiving, and that we all confuse with the "Founding Fathers") actually condemned the celebration of Christmas. It was actually BANNED in parts of the Colonies at one time.
I know many Christians know the origins of the holiday, but celebrate it anyway, out of a respect for tradition. That's fine. That's the same reason many atheists celebrate. It's tradition. It's the ignorant Evangelical types who actually believe they're celebrating Jesus' birthday that get on my nerves. Most of them don't even really understand their own holy book, to begin with, and just pick and choose the parts that are important to them ... speaking of which: anyone want to point out to me where in the Bible it says to celebrate Jesus' birth? Yeah, I thought not.
There IS one description of decorating a tree in the Bible, but it's only mentioned by way of condemning the practice as offensive to God.
So, anyway, as an atheist, I feel perfectly fine celebrating the solstice, Christmas, whatever. Personally, I call it Festivus (because I loved Seinfeld, and The Strike was one of my favorite episodes) and will be celebrating on December 23rd. Most of my family are some variation of non-denominational Christian, but they know the holiday has no actual Christian meaning, so it's a time for the extended family to get together and enjoy one another's company while we eat and exchange gifts. To us, it's about family togetherness and generosity.
Forget all the "Christ in Christmas" hooey.
May as well start telling Jews to start putting the Han back in Hannukah, or some such. (Maybe that could be somehow related to the Wookiee Life Day?)
Anyway, a Happy Whatever the Hell Holiday You Celebrate to Everyone! ^_^
~Cheers!