One would think that if anyone’s date of birth were remembered exactly, it would be that of our Savior, Jesus Christ. Unfortunately, the gospels do not pinpoint the date of Christ’s birth. The reason is probably that the focus of the gospels is on the kerygma or mystery of redemption– the passion, death, and resurrection of Christ. This focus is also probably why St. Mark’s Gospel does not even include the Christmas story but begins with the Baptism of the Lord at the River Jordan. Easter, on the other hand, can be better dated because of its linkage with Passover.
Prior to the legalization of Christianity by the Emperor Constantine in the year 313, no universal date or even formal celebration of Christmas is found. For instance, Origen (d. 255), St. Irenaeus (d. 202), and Tertullian (d. 220) do not include Christmas or its date on their lists of feasts and celebrations.
After legalization, the Church was better able to establish universal dates for feasts and to organize their public celebration. Moreover, we now see the Church addressing controversies concerning Jesus as true God and true man, and how He entered this world. Such concern about the mystery of the incarnation would focus more attention on the importance of celebrating Christmas, the birth of our Lord.
On the more “practical” side of this issue, Roman pagans used to gather at the hill where the Vatican is presently located to commemorate the “Birth of the Unconquered Sun.” This pagan feast was celebrated throughout the Empire either on December 25th (according to the Julian Calendar) or on January 6th (according to the Egyptian calendar). Although not proven with certainty, some historians credit Constantine, who declared Sunday as a day of rest in the Empire, with replacing the pagan festival with that of Christmas. Interestingly, since the 200s, Jesus was honored with the title, “Sun of Justice.”
Somehow all of these elements converged to the formal celebration of Christmas on December 25th. For instance, Christmas was celebrated in Rome by Pope Liberius (352-66) on December 25th. On December 25, 379, St. Gregory Nazianzus (d. 389) preached a Christmas sermon in Constantinople. In the Cathedral of Milan, St. Ambrose (d. 397) celebrated Christmas on December 25. Therefore, by the year 400, generally, the birth of Christ was set on December 25th with the exception of Palestine, where it was celebrated on January 6th until the mid-600s when it was then transferred to December 25th.
As an aside, the Feast of the Epiphany also emerged in Gaul (the Roman province of present day France) about the year 361. This feast was moved to January 6th which remains the official date.
While the concern for exact dating may preoccupy us at times, the most important point is celebrating the birth of our Lord and meditating on His incarnation. Remember that the title Christmas is derived from the Old English title Cristes Maesse which means “The Mass of Christ.” Each time we celebrate Mass and receive the Body and Blood of our Lord in the Holy Eucharist, we celebrate Christmas, when He, true God, was born as true man.