When was Jesus born in Bethlehem? The Bible describes with great certainty the fact of Christ’s birth as well as the place. But not the exact date.
One of the greatest of the Patristic writers, Basil of Caesarea believed Christ was born on November 20th in the year 4 BC. Another, Clement of Alexandria, speculated that Christ was born on November 17 in the year 3 BC. Still others, such as John Chrysostom, speculated that since shepherds were in the field the night Christ was born, it must have been in spring or summer. Similarly, Athanasius argued for a date of May 20. Cyril of Jerusalem reasoned for the date to be on either April l9th or 20th. And Ambrose of Milan made a strong traditional case for March 25th. Quite obviously though, no one really knew with any degree of certainty.
In 354, the Bishop of Rome started to observe December 25th as the date of Christ's birth. Four major Roman festivals had long been held in December, including Saturnalia which celebrated the returning sun god. As men converted from Paganism to Christianity, their culture was likewise gradually converted. Thus, it was natural for them to want to replace their old Pagan festivities with a celebration of the advent of their Savior.
In fact, Christians had already begun celebrating the incarnation and nativity of the Lord Jesus on that day since at least the early part of the third century--just a few generations removed the days of the Apostles. Thus, by 336, when the Philocalian Calendar--one of the earliest documents of the Patriarchal church--was first utilized, Christmas Day was already a venerable and tenured tradition. Though there is no historical evidence that Christ was actually born on that day--indeed, whatever evidence there is points to altogether different occasions--the conversion of the old Pagan tribes of Europe left a gaping void where the ancient winter cult festivals were once held. It was both culturally convenient and evangelically expedient to exchange the one for the other. And so joy replaced desperation. Celebration replaced propitiation. Christmas Feasts replaced new Moon sacrifices. Christ replace Baal, Molech, Apollo, and Thor.
Like so many calendar dates, the many different customs of Christmas ultimately melded together. And thus emerged the traditional Advent season--a four week long anticipation of Christmas beginning in late November and marked by a series of important feasts, fasts, rituals, and rites all the way through the designated day for the incarnation itself.