The pagan Roman celebration at Winter Solstice, or the Christian birthday party for Jesus?
Calculating Christmas pulled together some Biblical and historical sources and writes that, rather than our feast of Christ's mass being at attempt to baptize an existing feast, it was in fact something totally different.
Nathan wrote something in the other direction in his long and well written post Christ Be our Light.
I think that it doesn't really matter in the long run which came first - but rather that we celebrate what is truly important about this season - that God became Man, and dwelt among us - that the Word became Flesh - and that by His birth, death, and resurrection we can be re-united with the Father.
Have a blessed Christmas, all of you.