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.”
The notion that creation and redemption should occur at the same time of year is also reflected in ancient Jewish tradition, recorded in the Talmud.
The Babylonian Talmud preserves a dispute between two early-second-century C.E. rabbis who share this view, but disagree on the date:
Rabbi Eliezer states: “In Nisan the world was created; in Nisan the Patriarchs were born; on Passover Isaac was born … and in Nisan they [our ancestors] will be redeemed in time to come.” (The other rabbi, Joshua, dates these same events to the following month, Tishri.)
Thus, the dates of Christmas and Epiphany may well have resulted from Christian theological reflection on such chronologies: Jesus would have been conceived on the same date he died, and born nine months later
(I'd like to mention that 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).)