Notice how December 25th wasn’t even being considered in this conversation. Contrast this with the next time we see this discussion in detail, 100 years later, where the Roman almanac reads on December 25th: natus Christus in Betleem Judeae, or “Christ was born in Bethlehem of Judea.”
The missing piece
So what exactly is missing here? Well the missing piece of information comes in the form of what I pointed out earlier: there is a correlation between Christ’s birthday and Christ’s crucifixion. And that correlation is that our earliest documentation on calculating December 25th comes from the belief by early Christians that Christ was conceived on the same calendar day as his crucifixion. Today, very few modern Christians even are aware that there is a commemoration of Jesus’ conception, called “Annunciation.” And that day was very well known in the early days of Christianity. That day, is March 25th.
When was Jesus born? 3 + 9 = 12?
Tertullian, around 200 C.E. explains in depthhow we know the date based upon the Apostle John’s recording of Jesus’ crucifixion being on the 14th of Nisan. John here is using the Hebrew calendar, which by Tertullian’s explanation equates to the Roman calendar date of March 25th. Since Jesus’ deathday and Jesus’ conception were the same calendar day, then calculating Jesus’ birthday is simple as one must simply add the gestation period of a human baby to that date in order to arrive at a birthday. March 25th plus nine months gives us December 25th.
And as it turns out, other Patristics were aware of this calculation as well. And held to it, corporately for the whole of the Early Christian era. The great Augustine of Hippo writes in his famous On the Trinity:
“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.”
December 25th – not a pagan date
So you see, December 25th dear reader, is a date that was not adopted due to pagan worship of sun gods. December 25th is not an imaginary placeholder concocted by the early church in order to win over wily and wayward souls. December 25th is a widely agreed upon and ancient tradition that is sourced in both Scripture and the Patristics.
Confidence in the date of Christmas
We can come to an answer to the question, “When was Jesus born?” Of all of the Christmas traditions available to Christians to celebrate, the celebration of Christ’s birthday on December 25th, is one that you should feel confident in pursuing with boughs of holly and joyful delight.
…although the origins of using boughs of holly to decorate for Christmas are another story. But I will leave that for another day. For now, I will be celebrating with you on December 25th!
Further reading on dating Christ’s birth
For a more in depth analysis, see the work of Andrew McGowan in his piece “How December 25 Became Christmas” here.
 The Philocalian Calendar.
 Tertullian, Adversus Iudaeos 8-9.
 John 19:14, 19:31, 19:42.
 Augustine, Sermon 202.