Why December 25?
It's very tough for us North Americans to imagine Mary and Joseph trudging to Bethlehem in anything but, as Christina Rosetti memorably described it, "the bleak mid-winter," surrounded by "snow on snow on snow." To us, Christmas and December are inseparable. But for the first three centuries of Christianity, Christmas wasn't in December—or on the calendar anywhere.
If observed at all, the celebration of Christ's birth was usually lumped in with Epiphany (January 6), one of the church's earliest established feasts. Some church leaders even opposed the idea of a birth celebration. Origen(c.185-c.254) preached that it would be wrong to honor Christ in the same way Pharaoh and Herod were honored. Birthdays were for pagan gods.
Not all of Origen's contemporaries agreed that Christ's birthday shouldn't be celebrated, and some began to speculate on the date (actual records were apparently long lost). Clement of Alexandria (c.150-c.215) favored May 20 but noted that others had argued for April 18, April 19, and May 28. Hippolytus (c.170-c.236) championed January 2. November 17, November 20, and March 25 all had backers as well. A Latin treatise written around 243 pegged March 21, because that was believed to be the date on which God created the sun. Polycarp (c.69-c.155) had followed the same line of reasoning to conclude that Christ's birth and baptism most likely occurred on Wednesday, because the sun was created on the fourth day.
The eventual choice of December 25, made perhaps as early as 273, reflects a convergence of Origen's concern about pagan gods and the church's identification of God's son with the celestial sun. December 25 already hosted two other ...