One of the favorite arguments by the anti-Christmas folk, both the secularists and the ultraconservative, is that Christmas is “really” a pagan holiday. The neopagans I know particularly like it. Of course it ignores the fact that if a group decides to choose a date to celebrate something, whether or not someone else also celebrates something does not detract from its significance to that group or to those associated with that group. The “it’s really a pagan holiday and you Christians shouldn’t be celebrating the birth of Christ” canard is really a distractor for the more general antichristian agenda of removing recognition of Christian faith from public discourse altogether.
But it turns out that a bit of historical effort shows that even this canard is not based on fact. The “pagan origin” argument of Christianity has traditionally been a tool of anticlerics who have ignored the real history. In fact, the Aurelian establishment of the Roman festival of the “Birth of the Unconquered Sun” co-opted the already established Christmas celebration, not the other way around.
For an historical account of the establishment of the date, see this article by William Tighe. See also this article, which notes that Hippolytus of Rome also provided an earlier determination of the date.
Hat tip to Get Religion and, again, Jollyblogger.