Christmas is a celebration of the birth of Jesus Christ
The fifth century acceptance coincided with a decline in heathen worship and the adaptation of harmless activities into rich Christian symbolism. These included Saturnalia,the great Roman holiday in remembrance of the supposed "Golden Age"; Sigillaria, the Feast of Dolls, in which dolls and other toys, mostly earthenware, were given to children; and Brumalia, otherwise Dies Natalis Invicti Solis, The Birthday of the Unconquered Sun, when the days became longer after the solstice. Finally, there was Kalende Januarii, the New Year's Day, when everyone exchanged gifts and also tied in Juvenilia, the special festival of Childhood and Youth.
All of these days seemed to easily come together into one big Christian celebration and their ancient significance crossed over into the light of the Gospel.
But the year of Christ's birth raises question just as does the day. It, too, is not a definite. The 753rd year A.U.C. ( Anno Urbis Condita- -from the building of the city, i.e. Rome) is agreed upon as the traditional date. But, that is too late if we look at the Gospel of Matthew which distinctly affirms that "Jesus was born in Bethlehem of Judea in the days of Herod the king". Herod died in A.U.C. 750. It suggests that some time must have elapsed between the birth of Christ and the death of Herod for there to have been a visit from the wise men, the retreat to Egypt, and the Slaughter of the Innocents. The Gospel of Luke raises still another matter. It is not clear whether Tiberius Caesar's fifteenth year is counted from A.U.C. 765, when he was connected with Augustus in the Empire, or from the death of Augustus in A.U.C. 767. The real meaning of his remark about the census is not known (Luke 3:1). His information about the Nativity places it around A.U.C. 749 to 753. Matthew's account of the "Star in the East" and then over Bethlehem has been called an atmospheric meteor and at best suggested to astronomers that someone born in Judea at that time was destined for greatness.
We are nowhere told that Christ was born exactly at the time when the "star" appeared; but it is safe to say that His birth took place some time between the middle of A.U.C. 747 and the end of A.U.C. 749, i.e. 7 B.C. and 5 B.C.
The previously mentioned March 25th day was the pagan festival of spring. The church adopted this date as that of Mary's visit by the angel Gabriel, and added nine months to it to come of with December 25th as the day of Jesus' birth. Christ Mass, later called Christmas, was first celebrated in the year 354, December 25th, according to several sources.