Just why Christmas Day falls on Dec. 25 was originally only a question for the Western (Latinate) Church because Christmas fell on Jan. 6 for the Eastern Orthodox Church, which later adopted the Dec. 25 date. Even so, Jan. 6,
... is still the date of the celebration for the Armenian Apostolic Church and in Armenia, where it is a public holiday. As of 2012, there is a difference of 13 days between the modern Gregorian calendar and the older Julian calendar. Those who continue to use the Julian calendar or its equivalents thus celebrate December 25 and January 6 on what for the majority of the world is January 7 and January 19. For this reason, Ethiopia, Russia, Ukraine, Serbia, the Republic of Macedonia, and the Republic of Moldova celebrate Christmas on what in the Gregorian calendar is January 7; the Church of Greece celebrates Christmas on December 25.Christ's Mass (hence, Christmas) did not become an official festival day on the Roman Church's calendar until 300 years or so after Jesus. There was not much debate about placing it on Dec. 25. Modern historians used to argue that the Pope selected the date in order the Christianize a still-practiced Roman holiday celebrating the lengthening of daylight hours just after the winter solstice had passed. This idea has come under increasingly skeptical scrutiny, however, As Slate explains, (Why is Christmas in December?):
The reasoning goes that the growing church, recognizing the popularity of the winter festivals, attached its own Christmas celebration to encourage the spread of Christianity. Business historian John Steele Gordon has described the December dating of the Nativity as a kind of ancient-world marketing ploy. ...In fact, an early Christian Carthaginian scholar named Tertullian reported the calculation that the 14th of the Jewish month of Nisan (the day of the crucifixion according to the Gospel of John) in the year Jesus died 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, or the commemoration of Jesus’ conception. Thus, like important martyrs before him, Jesus was believed to have been conceived and crucified on the same date of the year, March 25, meaning that exactly nine months after the date of his conception, Dec. 25, Jesus was born.
This alternative explanation is sometimes deployed to dismiss the notion that the holiday had pagan roots. In a 2003 article in the journal , for example, historian William Tighe called the pagan origin of Christmas “a myth without historical substance.” He argued at least one pagan festival, the Roman Natalis Solis Invictus, instituted by Emperor Aurelian on Dec. 25, 274, was introduced in response to the Christian observance. The pagan festival “was almost certainly an attempt to create a pagan alternative to a date that was already of some significance to Roman Christians.” According to Tighe, the pagans co-opted the Christian holiday, not the other way around.
So, ascription of Dec. 25 to Jesus' birth took place 74 years before Emperor Aurelian instituted a Roman, pagan celebration of the winter solstice. So we can cast aside the idea that Christmas' dating was to take over an existing pagan holiday. There was, at the time, no pagan holiday to take over.
But wait! as they say on TV. There's more! Further complicating the debate is that there seem to be mathematical reasons for the Dec. 25 date that rest upon modern computational science, not legend.
A fascinating analysis of just what was the star that led the wise men to Bethlehem is given at Bethlehemstar.net. Here is my summary.
After Jesus was born, wise men, or Magi, from the east made their way to Judea. Being astronomers, they had read the stars and concluded that a new king had been born to the Jews. This is related in Matthew chapter 2.
No historian today claims that there nothing happened in the sky that corresponds, somehow, to what the wise men saw. Just what it was has been a scientific quest for about 400 years, since Johannes Kepler developed the first mathematical description of how the heavens worked. Kepler, whose equations are still used by NASA and astronomers around the world, himself spent may laborious hours trying to calculate the position of the planets and stars above the ancient Near East in the year of Jesus’ presumed birth, 6 BC. But he found nothing.
Since Kepler, many others have suggested that the star Matthew describes might have been a comet or a supernova, but there are no records of such events at this time anywhere in the ancient world, especially in China, whose astronomers were detailed and meticulous record keepers.
Jesus was presumed to have been born in 6 BC based on a book by the ancient Jewish historian Josephus, whose book, Antiquities, says that Herod died in 4 BC. Since clearly Jesus was born in the time of Herod, Jesus had to have been born before 4 BC, a year or two before.
But in fact, Herod did not die until 1 BC. This date is in fact what Josephus wrote, and is so stated in manuscripts of his book dated earlier than 1544. It was in 1544 that Antiquities was first set to the printing press. In that first edition, the typesetter erroneously set the wrong year of Herod’s death, and this edition became the standard from which all subsequent editions were made, including the one Kepler used.
Computers today solve Kepler’s equations in a snap. And what astronomers now know is that in September, 3 BC, the planet Jupiter came into conjunction with the star Regulus. That is, when viewed from the earth, Jupiter and Regulus appeared to touch or come very close together.
Jupiter is the largest planet. The ancients called it the king of planets. Regulus was called Rex by the Romans, Latin for king. In Persian its name was Sharu, which also meant king. To an ancient astronomer, for the king of planets and the king of stars to come together would have been weighted with portents. But Jupiter and Regulus did this not merely once, but three times over the course of the next year.
After appearing to touch Regulus, Jupiter’s path moved beyond. But after a few months, earth caught up with and passed Jupiter in its orbit. Jupiter then appeared to move backwards in the sky. This movement is called retrograde. All planets’ paths retrograde when seen from earth, that’s why they are called planets, which is Greek for “wanderer.” So Jupiter went back and touched Regulus again. Then the earth moved on and by September of 2 BC, Jupiter had retrograded once more and had touched Regulus a third time.
To astronomers as skilled as those of Babylonia heritage and learning, which the wise men almost certainly were, this three-time conjunction of the king of planets with the king of stars would have started them packing. But why did they decide that the Jews had anything to do with it?
All three conjunctions took place within the backdrop of the constellation Leo, the lion. The lion is the symbol of the Jewish tribe of Judah, which takes it name from a son of Jacob named Judah. In Genesis chapter 49, Jacob gives his son, Judah, the lion as his symbol and then dictates that only Judah’s descendants shall provide the rulers for subsequent generations. King David was a member of the tribe of Judah and so was Jesus’ father, Joseph, according to Matthew chapter one.
The wise men were obviously conversant with the relationship of lions with the tribe of Judah and Judah’s parentage of all the kings of Israel. The wise men may well have been (though probably weren't) Jews themselves, since a thriving Jewish colony remained in Iraq until a generation ago even though the Jews’ captivity in Babylon was ended in 538 BC. The kingly conjunctions of Jupiter and Regulus within the constellation of the lion were all the wise men needed to start heading toward the Roman province of Judea, which we know as Israel.
But what they did not know was just where the new king was born. So they stopped at the palace of the Roman vassal, King Herod, to inquire. Herod’s counselors quoted a prophecy from Micah that said the Messiah would be born in Bethlehem.
At Herod’s deceitful urging, the wise men went to Bethlehem, only five miles from Jerusalem. In the sky, Jupiter had just begun a third retrograde, this one, however, not to be followed by a conjunction. The thing about planetary retrogrades is this: just as a planet appears to reverse direction, it seems to stand still in the sky.
Here is what computers using Kepler’s equations show. Jupiter’s full stop for this retrograde took place on December 25, 2 BC. If you had been in Jerusalem on that evening, you would have seen the kingly planet Jupiter motionless in the sky almost due south, directly above Bethlehem. And better yet, its stationary position was in the middle of the Constellation Virgo, the Virgin.
However, many of the detailed explanations of these planetary and stellar phenomena, such as this one, do not seem to account for the fact that Dec. 25 on our calendar today is an altogether different date in the ancient world, as I briefly referenced above. December 25 on our calendar is 13 days further along than on the Roman Julian calendar used at the time the Pope officially designated Dec. 25 as Christmas Day. The original Dec. 25 date then corresponds (as above) to Jan. 7 on our calendar. So to be historically picky about it, the Russian church, for example is celebrating the date correctly on Jan. 7.
But wait! There's more! When modern astronomers say that Jupiter's "full stop" occurred on Dec. 25, 2 BC, which Dec. 25 do they mean, Gregorian (modern) or Julian (ancient)? They mean modern. If Jupiter's progression/retrogression transition was in process on Dec. 25 Gregorian, that means that the Magi arrived in Bethlehem on Dec. 12 Julian.
They did not get there the day Jesus was born. We know from Matthew that Jesus was born in a barn and laid by his parents in a manger, or feeding trough, after birth. Yet Matthew 2 clearly states, "On coming to the house, they saw the child with his mother Mary, .. ."
How old was Jesus by then? We have insufficient information to know. Presumably, Joseph, Mary and Jesus could have moved from the barn to the house the day after Jesus was born. It's not far fetched to imagine that room for a newborn and parents was made somehow.
The Magi did not go back to Herod. Herod, never one to countenance potential rivals to his throne (he had even executed his own sons), ordered soldiers to Bethlehem:
[H]e was furious, and he gave orders to kill all the boys in Bethlehem and its vicinity who were two years old and under, in accordance with the time he had learned from the Magi.In my view, the best interpretation of this narrative is that the Magi had no idea specifically when Jesus was born, but based on the sequence of astronomical observations, above, concluded it had to have been within the two years prior to their arrival at Herod's court. Another key may be that Matthew quotes the Magi as referring to Jesus as "born king of the Jews," not "newborn" king of the Jews. Hence, Herod's order to slaughter boys up to the age of two.
It is important, from an historical and biblical perspective, though not necessarily from a practical one for the Church, to confirm Dec. 25 (Julian, anyway) as the date the Wise Men got to Bethlehem, not the date Jesus was born. That date, I'm afraid, will likely forever remain unknown.
End note: David A Weintraub, Professor of Astronomy, Vanderbilt University, explains how Matthew's account makes use of some ancient astronomical and astrological (not very separate disciplines back then) terms that help us understand what the star narrative means; "Can astronomy explain the biblical Star of Bethlehem?"