While I haven't researched the topic thoroughly, it seems that I read somewhere that Christmas was first celebrated as an alternative to a pagan holiday. From what I understand, it's more than likely that Jesus wasn't born on December 25 at all... but that some pagan religion celebrated the birth of their god around the time of the winter solstice, so the Christians in the area revised the celebration to recognize Jesus as the true God. I've heard of Christians who don't celebrate Christmas at all because of that... but I kinda like the notion myself.
The main thing is that we are celebrating the coming of the Christ-child, who came to dispel the darkness. The date isn't important to me. But the Light is!
Whereby the dayspring from on high hath visited us,
to give light to them that sit in darkness and in the shadow of death,
to guide our feet into the way of peace.