Depends on what you class as Christmas. There were a number of festivals around the same time, some of which are still celebrated separately in the west by followers of various religions. They've gradually all melded into one great big commercial excuse for a shindig and become christmas.
It began with JC and ended with PC. Just think, you'll probably be on this planet when Christmas is officially replaced by an Islamic festival. We'll all have to stand around ritually burning Christmas trees, and will be stoned to death if we display so much as a sprig of misteltoe.