For the first 300 years of Christianity, it wasn't. When was Christmas first celebrated? In an old list of Roman bishops, compiled in A. D. 354 these words appear for A.D. 336: "25 Dec.: natus Christus in Betleem Judeae." December 25, Christ born in Bethlehem, Judea. This day, December 25, 336, is the first recorded celebration of Christmas.
For the first three hundred years of the church's existence, birthdays were not given much emphasis - not even the birth of Christ. The day on which a saint died was considered more significant than his or her birth, as it ushered him or her into the kingdom of heaven. Christ's baptism received more attention than his birthday in the January 6 feast of Epiphany.
No one knows for sure on what day Christ was born. Dionysus Exiguus, a sixth century monk, who was the first to date all of history from December 25, the year of our Lord 1. Other traditions gave dates as early as mid-November or as late as March. How did Christmas come to be celebrated on December 25? Cultures around the Mediterranean and across Europe observed feasts on or around December 25, marking the winter solstice. The Jews had a festival of lights. Germans had a yule festival. Celtic legends connected the solstice with Balder, the Scandinavian sun god who was struck down by a mistletoe arrow. At the pagan festival of Saturnalia, Romans feasted and gave gifts to the poor. Drinking was closely connected with these pagan feasts. At some point, a Christian bishop may have adopted the day to keep his people from indulging in the old pagan festival.
Historian William J. Tighe offers a different view, however. When a consensus arose in the church to celebrate Christ's conception on March 25, it was reasonable to celebrate His birth nine months later.
Adapted from ChristianHistoryTimeline.com.