The story goes something like this: Christmas is celebrated on December 25th because newly-converted Constantine (emperor of Rome in AD 306–337) tried to Christianize a pagan holiday in the hopes of making Christianity more palatable to the Romans. He appointed the date of the Feast of the Unconquered Sun, December 25th, to celebrate the birth of Jesus. So, the Christmas holiday is just a copycat of a pagan holiday.
This is the story. Is it true, though? If not, how did Christmas come to be celebrated on December 25th?
Did Constantine Choose December 25th?
The biblical text doesn’t offer much detail with respect to the date of Jesus’ birth. There’s no date given in the Gospels or Acts. Luke mentions shepherds in the fields with sheep (Luke 2:8), implying it might have been spring as opposed to winter, but that’s hardly sufficient evidence for or against a December 25th date.
What about extra-biblical sources? Is there any evidence for a birth celebration in the writings of the church fathers? Origen (AD 165–264) wrote mockingly of Romans celebrating the anniversaries of birthdays, dismissing them as altogether “pagan.” Clement (AD 150–215) wrote about the birth of Jesus, reporting several different dates proposed by different Christian groups. There’s no mention of December 25th, though.
The first time we see the December 25th birthday in written history is in the Philocalian Calendar from AD 354. The document contains various dates tied to events, the first being December 25th. Next to the date, it says, “Christ was born in Bethlehem of Judea.” Now, this is after the reign of Constantine, leaving open the possibility of a co-opted pagan holiday. But there’s more.
In AD 400, Augustine (AD 354–430) wrote about the Donatists, a group of Christians who emerged in AD 312. These dissident Christians seem to have been the first to celebrate the nativity of Jesus on December 25th. Being under Roman oppression, the Donatists were opposed to any compromise with anything pagan. They wouldn’t have merged Christmas with a pagan, Roman holiday.
Not only this, but in the article “How December 25 Became Christmas,” Andrew McGowan says, “The December 25 feast seems to have existed before 312—before Constantine and his conversion.” According to McGowan, the earliest known celebrations of Christmas can actually be traced to older North African traditions from the second half of the third century. These traditions are believed to have been adopted by the Donatists.
So, the earliest known celebration of Christmas on December 25th was in the second half of the third century. This is well before Constantine, during a time when Christians were persecuted in Rome and vigilant in avoiding any pagan connections.
Where Did the Idea of a Christianized Pagan Festival Come From?
It seems clear that the story about Constantine taking over a pagan celebration for Jesus just isn’t true. So, where and when did the idea that Christmas grew out of pagan origins come from?
We’ve seen that the church fathers didn’t mention December 25th as being Jesus’ birthday, so of course they never mentioned co-opting a pagan holiday to commemorate it. But if it really were the case that the date of a pagan holiday was chosen for Christmas in order to make Christianity more popular and understandable, surely one of them would have written about it. It’s not like the idea of pagan celebrations being adopted by Christians was controversial. Christian leaders often did this. In AD 601, Gregory the Great urged Christians to turn pagan temples into churches and to repurpose pagan festivals into feasts celebrating Christian martyrs.
Needless to say, there are no known early sources indicating Christmas grew out of pagan origins. For this, we have to go all the way to the 12th century. In his article, McGowan points out Dionysius bar-Salibi was the first to suggest Christmas was moved to December 25th to correspond with the pagan festival Sol Invictus (i.e., the Feast of the Unconquered Sun). Then the record is silent again.
The idea that early Christians commandeered pagan winter solstice celebrations for their own purposes actually comes from post-enlightenment comparative religion scholars. For the first 1700 years of the church, no one made that connection.
Why Did the Early Christians Celebrate on December 25th?
So, how did Christians end up celebrating Christmas on December 25th? It’s widely accepted that the reason December 25th is the date for Christmas has to do with the date of Jesus’ death and his conception. To answer this question, we need to do some math.
There are three dates in our equation: Jesus’ death, Jesus’ conception, and Jesus’ birth.
Let’s start with the date we’re most sure of: Jesus’ death. Easter developed as a Christian holiday much earlier than Christmas, so we know a little more about it. Tertullian (AD 155–220) said Jesus died on the 14th day of Nisan. This is March 25th according to the calendar we’re familiar with, and it lines up with biblical evidence. Notice March 25th is nine months before December 25th.
Our second date is Jesus’ conception. An anonymous fourth-century Christian treatise titled On the Solstices and Equinoxes ties the date of Jesus’ death to the date of Jesus’ conception:
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.
In On the Trinity, Augustine ties the two dates together, as well, but goes a step further:
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.
Knowing human gestation takes nine months, and believing Jesus was conceived on March 25th, Augustine did the math for us. If Jesus was conceived on March 25th, then nine months later, on December 25th, he was born. This is how we’ve come to celebrate Jesus’ birthday on December 25th.
In the end, we can’t be certain of the date of Jesus’ nativity. That’s okay. We can be certain of something much greater. Christmas reminds us that the King of Ages came obscurely, miraculously, and mysteriously. Christmas represents the dawn of redeeming grace and the mysteries at the center of the world. A virgin birth. A perfect life. A rugged cross. A bodily resurrection. These are not fanciful Christmas stories based on pagan myths. This is the way the world really is. This is what Christmas is about.
 Origen, Homily on Leviticus 8