From the Treasury of Daily Prayer:

The exact date of the birth of Jesus is not known, and during the earliest centuries of the Church it seemed to have little significance. This followed the Early Church’s tradition of honoring and celebrating a Christian’s death as his or her birth date into eternity and the ongoing presence of Jesus. Likewise the life, work, death, and resurrection of Christ was of much greater importance to early Christians than the early details of His life. The earliest nativity feast, Epiphany (January 6), celebrated both the birth and Baptism of Christ. However, in the fourth century great Christological controversies that questioned Christ’s divinity and humanity raced throughout Christianity. By AD 336, December 25 had been established in Rome as the celebration of Christ’s birth, a festival welcomed particularly by orthodox Christians in the West. From Rome, Christ’s natal festival spread throughout the Western Church. In Eastern traditions of the Church, Epiphany remains the principal celebration of the birth of Jesus.