You can find lots of intriguing history in this deciphering of the most famous holy day on the Christian calendar.

A snip:

Around 200 C.E. Tertullian of Carthage reported the calculation that the 14th of Nisan (the day of the crucifixion according to the Gospel of John) in the year Jesus diedc was equivalent to March 25 in the Roman (solar) calendar.  March 25 is, of course, nine months before December 25; it was later recognized as the Feast of the Annunciation–the commemoration of Jesus’ conception.  Thus, Jesus was believed to have been conceived and crucified on the same day of the year. Exactly nine months later, Jesus was born, on December 25.

This idea appears in an anonymous Christian treatise titled On Solstices and Equinoxes, which appears to come from fourth-century North Africa. The treatise states: “Therefore our Lord was conceived on the eighth of the kalends of April in the month of March [March 25], which is the day of the passion of the Lord and of his conception. For on that day he was conceived on the same he suffered.”  Based on this, the treatise dates Jesus’ birth to the winter solstice.

Augustine, too, was familiar with this association. In On the Trinity (c. 399-419) he writes: “For he [Jesus] is believed to have been conceived on the 25th of March, upon which day also he suffered; so the womb of the Virgin, in which he was conceived, where no one of mortals was begotten, corresponds to the new grave in which he was buried, wherein was never man laid, neither before him nor since. But he was born, according to tradition, upon December the 25th.”

In the East, too, the dates of Jesus’ conception and death were linked. But instead of working from the 14th of Nisan in the Hebrew calendar, the easterners used the 14th of the first spring month (Artemisios) in their local Greek calendar–April 6 to us. April 6 is, of course, exactly nine months before January 6–the eastern date for Christmas. In the East too, we have evidence that April was associated with Jesus’ conception and crucifixion. Bishop Epiphanius of Salamis writes that on April 6, “The lamb was shut up in the spotless womb of the holy virgin, he who took away and takes away in perpetual sacrifice the sins of the world.”13 Even today, the Armenian Church celebrates the Annunciation in early April (on the 7th, not the 6th) and Christmas on January 6.

Thus, we have Christians in two parts of the world calculating Jesus’ birth on the basis that his death and conception took place on the same day (March 25 or April 6) and coming up with two close but different results (December 25 and January 6).

Connecting Jesus’ conception and death in this way will certainly seem odd to modern readers, but it reflects ancient and medieval understandings of the whole of salvation being bound up together. One of the most poignant expressions of this belief is found in Christian art. In numerous paintings of the angel’s Annunciation to Mary–the moment of Jesus’ conception–the baby Jesus is shown gliding down from heaven on or with a small cross (see photo of detail from Master Bertram’s Annunciation scene); a visual reminder that the conception brings the promise of salvation through Jesus’ death.

There’s much more, so check out the whole thing.