I just read a very interesting article about the origin of December 25 as the birthday of Jesus. A popular modern myth is that we've just taken over the pagan celebration of Saturnalia. This has certainly caught a lot of people's imagination, but at the time December 25 was first used Christians were still being persecuted by Rome and weren't in the habit of associating themselves with its holy days. The appropriation of pagan rites would come later, as the Church proselytized northern Europe. It also looms large in the imagination of us Protestants, who, accusing Catholics of practicing paganism never considered that the charge would be extended to us!
The article addresses this myth, but goes on to review ancient writers who believed that Jesus was conceived on March 25 and that He died on the same day, creating a cycle. Such cycles were common in Jewish scholarship. They believed the creation of the world, the birth and deaths of the patriarchs (Abraham, Isaac, and Jacob), and the Passover all happened on the same dates. Nine months after March 25 is December 25.