Christmas as a celebration must have been introduced during apostolic times. Irenaeus was the first who contended that the exact date of the birth of Christ was December 25. The earliest known Christmas celebration is recorded in a first century document called the Apostolic Constitutions. This manuscript, thought to have been written by Clemens Romanus around the year AD 70, is believed to record a celebration that occurred in the earliest days of Christianity. There is no mention of birth celebrations in the writings of early Christian writers such as Irenaeus (c. 130–200) or Tertullian (c. 160–225). Origen of Alexandria (c. 165–264) goes so far as to mock Roman celebrations of birth anniversaries, dismissing them as “pagan” practices—a strong indication that Jesus’ birth was not marked with similar festivities at that place and time. As far as we can tell, Christmas was not celebrated at all at this point. In about 200 C.E., a Christian teacher in Egypt makes reference to the date Jesus was born. According to Clement of Alexandria, several different days had been proposed by various Christian groups. Surprising as it may seem, Clement doesn’t mention December 25 at all. Clement writes: “There are those who have determined not only the year of our Lord’s birth, but also the day; and they say that it took place in the 28th year of Augustus, and in the 25th day of [the Egyptian month] Pachon [May 20 in our calendar] … And treating of His Passion, with very great accuracy, some say that it took place in the 16th year of Tiberius, on the 25th of Phamenoth [March 21]; and others on the 25th of Pharmuthi [April 21] and others say that on the 19th of Pharmuthi [April 15] the Savior suffered. Further, others say that He was born on the 24th or 25th of Pharmuthi [April 20 or 21].” 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. Indeed, the night from 5th to 6th January is the famous Twelfth Night, set as the background in the Shakespearean comedy with this title. From its earliest days, the Twelve Days of Christmas festival involved masked dancers and play actors who cavorted through the streets and visited homes unannounced to beg for holiday treats and drink. In England they were called "Mummers," from the French term "momer," which means to wear a mask. Some historians suggest that when the Christian church initially subsumed the pagan Saturnalia, it may have encouraged or tolerated demonstrations by the newly faithful mocking the old Roman gods. Those early revelers donned grotesque masks satirizing the Roman deities but their masked street antics ultimately became a popular and unstoppable part of the Christian Christmas festival. 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 Tbishops 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 Christian heresy, 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. In the East, January 6 (day of Epiphany) was at first not associated with the magi alone, but with the Christmas story as a whole. The birth of Jesus was celebrated in connection with the Epiphany, in order to emphasize the celebration of the baptism of Jesus. So, almost 300 years after Jesus was born, we finally find people observing his birth in mid-winter. But how had they settled on the dates December 25 and January 6? There are two theories today: one extremely popular, the other less often heard outside scholarly circles (though far more ancient). The most loudly touted theory about the origins of the Christmas date(s) is that it was borrowed from pagan celebrations. The Romans had their mid-winter Saturnalia festival in late December; barbarian peoples of northern and western Europe kept holidays at similar times. To top it off, in 274 C.E., the Roman emperor Aurelian established a feast of the birth of Sol Invictus (the Unconquered Sun), on December 25. Christmas, the argument goes, is really a spin-off from these pagan solar festivals. According to this theory, early Christians deliberately chose these dates to encourage the spread of Christmas and Christianity throughout the Roman world: If Christmas looked like a pagan holiday, more pagans would be open to both the holiday and the God whose birth it celebrated. Despite its popularity today, this theory of Christmas’s origins has its problems. It is not found in any ancient Christian writings, for one thing. Christian authors of the time do note a connection between the solstice and Jesus’ birth: The church father Ambrose (c. 339–397), for example, described Christ as the true sun, who outshone the fallen gods of the old order. But early Christian writers never hint at any recent calendrical engineering; they clearly don’t think the date was chosen by the church. Rather they see the coincidence as a providential sign, as natural proof that God had selected Jesus over the false pagan gods. It’s not until the 12th century that we find the first suggestion that Jesus’ birth celebration was deliberately set at the time of pagan feasts. A marginal note on a manuscript of the writings of the Syriac biblical commentator Dionysius bar-Salibi states that in ancient times the Christmas holiday was actually shifted from January 6 to December 25 so that it fell on the same date as the pagan Sol Invictus holiday. In the 18th and 19th centuries, Bible scholars spurred on by the new study of comparative religions latched on to this idea. They claimed that because the early Christians didn’t know when Jesus was born, they simply assimilated the pagan solstice festival for their own purposes, claiming it as the time of the Messiah’s birth and celebrating it accordingly. The 25-December Christmas was promoted in the Christian East as part of the revival of Nicene Christianity following the death of the pro-Arian Emperor Valens at the Battle of Adrianople in 378. The feast was introduced in Constantinople in 379, in Antioch by John Chrysostom towards the end of the fourth century, probably in 388, and in Alexandria in the following century. The feast disappeared after Gregory of Nazianzus resigned as bishop in 381, although it was reintroduced by John Chrysostom in about 400. Even in the West, January 6 as the celebration of the nativity of Jesus seems to have continued until after 380. Bibliography https://www.biblicalarchaeology.org/daily/biblical-topics/new-testament/how-december-25-became-christmas/