For many centuries, western Christians have celebrated the birth of Jesus Christ on December 25, while those of the Eastern Orthodox faiths have celebrated it a week later. How did experts determine the date? Andrew McGowan of Biblical Archaeology Review has some answers.
Scholars have long debated the actual date of Christ's birth, but most agree that it wouldn't have been December 25. Most also agree that the current date was chosen to parallel Pagan celebrations such as Saturnalia.
In the article, McGowan writes:
The earliest mention of December 25 as Jesus’ birthday comes from a mid-fourth-century Roman almanac that lists the death dates of various Christian bishops and martyrs. The first date listed, December 25, is marked: natus Christus in Betleem Judeae: “Christ was born in Bethlehem of Judea.” In about 400 C.E., Augustine of Hippo mentions a local dissident Christian group, the Donatists, who apparently kept Christmas festivals on December 25, but refused to celebrate the Epiphany on January 6, regarding it as an innovation. Since the Donatist group only emerged during the persecution under Diocletian in 312 C.E. and then remained stubbornly attached to the practices of that moment in time, they seem to represent an older North African Christian tradition.