How December 25 Became Christmas

If you haven't read this, you should, especially if you think you know the answer!

Here the link http://www.bib-arch.org/e-features/christmas.asp

Here's a quote.

The December 25 feast seems to have existed before 312—before Constantine and his conversion, at least. As we have seen, the Donatist Christians in North Africa seem to have know it from before that time. Furthermore, in the mid- to late fourth century, church leaders in the eastern Empire concerned themselves not with introducing a celebration of Jesus’ birthday, but with the addition of the December date to their traditional celebration on January 6.