Around the world, Christians anxiously await the arrival of Christmas, a joyous day to celebrate the birth of Jesus. But more than two millennia after Jesus’ momentous ministry, even Christians can’t agree on his birthday. In Catholic and Protestant traditions, Christmas is celebrated Dec. 25, while Orthodox Christians in countries like Russia, Greece and Egypt celebrate Christmas on Jan. 6 or 7.

Yet according to historians and biblical scholars, even those traditional dates are debatable. The Bible’s most detailed account of the Nativity is in the New Testament’s Gospel of Luke, but even that “orderly” narrative — complete with highly specific references to Roman rulers and a worldwide census — fails to name a day, month or even a year for Jesus’ birth.

“We have this modern obsession with dates and chronological order, but the gospel writers were much more interested in theology than chronology,” says Ian Paul, a theologian, biblical scholar and author who blogs at his website Psephizo.

That said, Paul’s own best guess for the true date of Jesus’ birth is somewhere in September, based on a complex set of calculations related to the birth of John the Baptist, also mentioned in Luke. A fall date for Christmas makes sense when you consider that the shepherds were in the fields tending their flocks, a sign of mild weather. Paul says that by December, the Judean foothills outside of Bethlehem are cold enough to get snow.

Ultimately, whether Jesus was born in December, September or March doesn’t change the true meaning of Christmas, but the debate over Jesus’ “real” birthday shows just how difficult it is to place specific dates on ancient events.

Jesus Wasn’t Born in ‘Year 1’

Before we even get to the month and day debate, historians generally agree that we’ve got the year of Jesus’ birth all wrong. How can that be, though, if “year 1” on the Gregorian calendar was based on the year that Jesus was born?

The short answer is that the man who invented the idea of anno Domini (shortened to A.D.) for “Year of Our Lord” was off by several years. Even Pope Benedict XVI agreed in a 2012 book that Dionysius Exiguus, the sixth-century monk who first calculated the year of Jesus’ birth, miscounted, and that Jesus was likely born anywhere between 7 B.C. and 2 B.C. (Modern writers may use C.E. in place of A.D. and B.C.E. in place of B.C., to be religiously neutral.)

One compelling reason for an earlier birth year is that the Bible mentions in several places that Jesus was born when Herod the Great was king of Judea. But Herod the Great supposedly died in 4 B.C.E., according to Flavius Josephus, the famed Roman-Jewish historian who lived in the first century C.E. If we take Josephus’s word for it, then Jesus must have been born at least four years earlier (and probably more) than our calendar says.

How December and January Became the Traditional Dates for Christmas

The popular theory that Christians chose Dec. 25 to co-opt the pagan solstice festival of Sol Invictus is not based on strong evidence, but rather on the margin scribblings of an unnamed Syrian monk in the 12th century. Rather than accusing Christians of stealing the holiday, he was offering a theory for why western churches “moved” Christmas from January to December.

In fact, the first mention of a date for Christmas was around 200 C.E., and the earliest celebrations of it were 250-300, “a period when Christians were not borrowing heavily from pagan traditions of such an obvious character,” according to the Biblical Archaeology Society.

For centuries after Jesus’ death, early Christians didn’t pay much attention to his birthday. In those days, Christians were persecuted and even martyred for their faith, which led them to put an emphasis on Easter, when Jesus himself was martyred on the cross, but overcame death and was resurrected.

It wasn’t until the third and fourth centuries C.E. that early Christian theologians put forth possible dates for Jesus’ birth. And even then, those dates were related to Easter. In ancient times, Paul says, there were traditions that the lives of great men were connected to specific times of year. Heroic figures often died in the same month and on the same day that they were born (years apart of course). In Jesus’ case, it looks like ancient sources believed that he was either born or divinely conceived during Passover, the springtime Jewish holiday during which Jesus was later crucified.

Christians who believed that Jesus was conceived around the time of Passover/Easter counted nine months ahead to identify his birthday. In Rome and other western locales, they calculated Passover in the year that Jesus died as occurring March 25. In eastern Christian communities, they used a Greek calendar that placed that same Passover on April 6. Add nine months and that’s how Christianity came up with two traditional dates for Christmas: Dec. 25 and Jan. 6.

The September Theory of Christmas

So why do biblical scholars like Ian Paul believe that the true date for Christmas ought to be in September? It comes from a close reading of the clues left behind in Luke, particularly what the gospel’s authors have to say about the timing of the birth of John the Baptist in relation to Jesus.

Luke’s version of the Christmas story doesn’t begin with Mary and Joseph, but with another couple, Elizabeth and Zechariah, who were old and childless. Zechariah was a priest in the Temple, and one day the angel Gabriel appeared to him in the Temple and told Zechariah that his wife would bear a son named John who would prepare the world for the coming of the Lord. Zechariah doubted Gabriel’s message and was stuck dumb. But when his service in the Temple was over, Zechariah went home and Elizabeth soon became pregnant.

What does this story have to do with Jesus? The angel Gabriel also visited Mary and told her that Mary was going to conceive and give birth to Jesus, the Son of God, even though she was a virgin. Luke tells us that this second visitation to Mary happened “in the sixth month of Elizabeth’s pregnancy.”

With that key fact, it’s possible to deduce that Jesus was conceived six months after John was conceived. But that only helps us if we know when exactly John was conceived. And how would we know that?

Again, the Bible provides more clues. Luke tells us that Zechariah “belonged to the priestly division of Abijah.” Each division of priests took turns performing sacrifices and other services in the Temple. In 1 Chronicles 24, the order of Temple service is laid out by divisions numbering one through 24, with Abijah listed as eight in the rotation.

As Paul calculated on his blog, if each priestly division served for one week with the first week of the ecclesiastical calendar landing in late March, that would put Zechariah in the Temple in early June. If Elizabeth conceived soon after the angel visited Zechariah in the Temple, and Mary conceived six months later, then it places Jesus’ birth in September of the following year.

Paul likes the September theory of Christmas for several reasons, including the shepherd idea mentioned above. Would Luke have placed shepherds in the fields if it was the middle of winter?

But there are also some holes in this theory. The biggest problem is that each priestly division served more than once a year in the Temple. What if Gabriel appeared to Zechariah during his second stint in the temple six months later? That would place Jesus’ birth in March, which Paul admits is a distinct possibility.

Thomas Wayment, is a professor of classical studies at Brigham Young University who has written about the competing theories regarding the timing of Jesus’ birth. He finds the debate over Jesus’ birthday is intellectually fascinating and worthy of discussion but misses the point spiritually.

“Maybe, just maybe, we’re better leaving it open in a sense,” he says. He has seen early Christian references to Jesus’ birth in April and May in addition to December and January. “We’re celebrating an event, not a day.”