Early Christians celebrated the Passion of Christ on April 6 (the solar equivalent of 14 Nisan). This Passion included the themes of the incarnation, atonement and resurrection of Jesus Christ. The Scriptures indicate that the incarnation of Christ began with his conception (Luke 1:35). Therefore, Christ’s death coincides with his conception. If you accept the date of April 6 for the conception and add 38 weeks for a typical human gestation period and you will conclude with a hypothetical birth date for the week of December 25.
“Therefore, our Lord was conceived on the eighth of the kalends of April in the month of March, which is the day of the passion of the Lord and of his conception. For on the day that he was conceived on the same he suffered.” (translation from De solstitia et aequinoctia conceptionis et nativitatis domini nostri iesu christi et iohannis baptistae, a tract circulated among early churches)
In A.D. 243, the date of December 25 was proposed for the celebration of the nativity in De pascha computus. The Donatist schism took place in A.D. 311, and one point of contention regarded the date of December 25 for the nativity. This disagreement about that date documents the fact that such a date was well accepted by A.D. 300. In a document entitled the Chronograph of 354, December 25 is listed as “VIII kal. Ian. natus Christus in Betleem Iudee.”
In A.D. 274, the Roman emperor Aurelian established December 25 as the date for the Natalis solis invicti, the Roman festival of the winter solstice. He chose this specific date to advance his own religious agenda: merging the revived cult of Sol into the Christian monotheistic faith that was spreading throughout his empire, threatening to eclipse the old Roman religions.
[based upon Thomas J. Talley, The Origins of the Liturgical Year, Liturgical Press; 2nd edition|January 1986]