From When you strip away all the pious fiction, what is left of the real Jesus?:
No one knows the exact date of the birth of Jesus Christ. December 25 was selected by the Western Church only in the 4th century to rival the pre-Christian Roman feast of the Victorious Sun. Nor was Christ born in Year 1 as the era bearing his name continues to pretend. The New Testament locates the event shortly before the death of King Herod which occurred in 4BC. A 6th-century Roman monk, Dionysius the Small, is guilty of the miscalculation: he wrongly placed the birth of Jesus in the year 753 ab urbe condita — after the foundation of the city of Rome — instead of 747 or 748.