Why December 25?

The origin of Christmas had nothing to do with paganism | Gene Edward Veith

According to conventional wisdom, Christmas had its origin in a pagan winter solstice festival, which the church co-opted to promote the new religion. In doing so, many of the old pagan customs crept into the Christian celebration. But this view is apparently a historical myth—like the stories of a church council debating how many angels can dance on the head of a pin, or that medieval folks believed the earth is flat—often repeated, even in classrooms, but not true.

William J. Tighe, a history professor at Muhlenberg College, gives a different account in his article "Calculating Christmas," published in the December 2003 Touchstone Magazine. He points out that the ancient Roman religions had no winter solstice festival.