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. ...But there seem to be mathematical reasons for the Dec. 25 date that rest upon modern computational science, not legend.
This alternative explanation is sometimes deployed to dismiss the notion that the holiday had pagan roots. In a 2003 article in the journal Touchstone, 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.
Update: 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. 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 to Jan. 7 on our calendar. So to be historically picky about it, the Russian church and others that still use the Julian calendar for religious dating are correctly celebrating the date 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 or so 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.
One clue, however, lies in that Herod directed the Magi to come back and report to him once they found the one whom the Magi had referred to as the "one who has been born king of the Jews."
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 their sequence of astronomical observations 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.