Short answer is: he was not born on Christmas. On this almost all who seriously study the Bible agree. I gave a longer explanation a while back. If you are really interested go to my profile and look at the questions I've answered.
It doesn't matter what exact day He was born. We celebrate the fact that He was born and even more so that He died and 3 days later was resurrected overcoming death (sin) so that we all can receive salvation from our sins through Him and His sacrifice. Have a Merry and Blessed Christmas.