The origins of December 25 as Christmas

An online article by Andrew McGowan, an associate professor of early Christian history at the Episcopal Divinity School in Cambridge, Massachusetts, for the Biblical Archaeology Review discusses how December 25 came to be chosen as the date of Christ's birth.

From the article:

Clearly there was great uncertainty, but also a considerable amount of interest, in dating Jesus’ birth in the late second century. By the fourth century, however, we find references to two dates that were widely recognized—and now also celebrated—as Jesus’ birthday: December 25 in the western Roman Empire and January 6 in the East (especially in Egypt and Asia Minor). The modern Armenian church continues to celebrate Christmas on January 6; for most Christians, however, December 25 would prevail, while January 6 eventually came to be known as the Feast of the Epiphany, commemorating the arrival of the magi in Bethlehem. The period between became the holiday season later known as the 12 days of Christmas.