Just hear those sleigh bells ring-a-ling

Last year, Garrison Keillor delivered a grumpy and borderline bigoted editorial about how Christmas belongs to the Christians and everybody else, specifically the Unitarians and the Jews, should "buzz off." Jews, in particular, should stop writing "dreck" (like White Christmas, I guess). I'm not 100% sure he wasn't being ironic, but I'm about 99.9%. And while I could rant on this all day, I have just a reflection instead.
What Keillor says struck me as odd because really, the Christians were just about the last ones to come to the holiday party, and about 99% (all right, I pulled that number out of nowhere, but I bet it's close) of the stuff we typically associate with Christmas comes from other faiths. While you could see this as an example of Christianity hijacking other faiths, I actually think it's kind of nice, the way cultures have melded around this time of year.
I heard a sermon once that whether we believe in Christmas or Hanukkah or Solstice or Festivus or just a day off work, what we all know in our hearts is that this time of year is sacred. Maybe it's intrinsically sacred, or maybe it's because we all decided it was, or maybe it's sacred because of the loved ones with whom we share it. And one thing we all seem to have in common is light. Christians celebrate Christ bringing light to the world. Folks who celebrate Solstice celebrate the fact that the days are from here on out will be longer - that the world will become bright again. Hanukkah is the festival of light. I'm certainly pro-light, whatever else.
Speaking of which, as somebody mentioned in the comments a while back, the only thing we know about the date of Christ's birth, really, is that it probably didn't happen on December 25th. The Bible doesn't make any mention. However, since the Bible says that shepherds were watching over their flocks by night, and since shepherds at the time only kept their watch at night was in the spring, it's not looking good for December. Now some Biblical scholars have said that the reason we celebrate on the 25th is that if you assume that god created light four days after the vernal equinox (and really, why would you think anything else?), then obviously, Christ was conceived on March 25th, meaning he'd have been born 9 months later on December 25th. But that seems a bit of a reach to me.
Most people figure that Christians started celebrating the birth of their lord in late December because they felt left out, what with all the other feasts and festivals at the time. Early Christians might even have been trying to compete with the cult of Mithra, who celebrated the birth of their infant god of light, in a cave or a stable, on the same day.
So I mentioned how few of the things we do at Christmas were originated by the Christians. For example, Santa Claus and all his other incarnations are probably descended from the god Odin in Germanic mythology, who flew through the sky at Christmas time delivering presents to people who have been good and punishments to people who have been bad. Odin was often depicted as an old dude with a long white beard. There's also a nice lady called The Grandmother in Italy who went around sticking presents inexplicably in people's socks. Martin Luther, in an effort to oust St. Nick, came up with Christkind. Christkind, said to represent the baby Jesus, skips the middle men and delivers presents himself. It's from Christkind that we get Kris Kringle. Incidentally, in most of Europe, Santa lives not at the North Pole but in the mountains of Korvatunturi in Lapland, Finland. North pole's better.
So Christmas trees originally belonged to the Druids, what with their tree worshiping. They also introduced us to garland and holly. Other pagans contributed the yule log, mistletoe, and many of the foods we associate with Christmas, like gingerbread men (probably descended from a Saturnalia tradition of eating people-shaped biscuits for no apparent reason). Gift-giving also happened at Saturnalia.
So there you have it - our lovely ecumenical coming together of customs to create one lovely megaholiday that doesn't belong at all to one group or another.

Most of this comes from a HowStuffWorks.com, but also Wikipedia and my brain.