DETROIT — There you are, joyfully basking in Christmas cheer, and some classmate, co-worker, friend or neighbor starts in on you about how Jesus couldn’t have been born on Dec. 25 because the shepherds wouldn’t have been in their fields with their sheep at that time of year.

And then he or she tells you that the reason the early Church chose Dec. 25 was because it was the same day as a pagan festival — and that this fact makes Christmas itself a pagan festival.

You’re either totally flummoxed by this argument, or maybe you try to explain how sometimes the early Church took some of the forms extant in the pagan world and Christianized them — like adapting Roman basilicas for liturgical use as churches.

That’s not a bad retort — even today, the Church practices “inculturation” in its missionary efforts — but it’s not the right answer, according to one contemporary scholar.

The choice of Dec. 25 resulted from “calendrical calculations that had nothing to do with pagan festivals,” argues William J. Tighe, an associate professor of history and faculty adviser to the Catholic campus ministry at Muhlenberg College in Pennsylvania, in a December 2003 article in Touchstone magazine.

Not only that, but the pagan festival of the “Birth of the Unconquered Sun” wasn’t instituted by the Roman emperor until 274 A.D., Tighe says, and was probably launched as a new pagan holiday to unite Romans as an alternative to a day of significance to Roman Christians — the opposite of the common argument that the pagan holiday came first.

Tighe calls the whole argument of the pagan origin of Christmas a “myth without historical substance.” He contends it arose from the mistaken ideas of two scholars of the late 17th and early 18th centuries — one a Protestant bent on showing how the fourth-century Church embraced “paganizations” that perverted pure apostolic Christianity, and the other a Benedictine monk arguing that the Church adopted pagan festivals for Christian purposes without paganizing the Gospel.

But the real story — argued in detail in Tighe’s article — has to do, first, with figuring out exactly when Christ died and was resurrected, and second, with a curious notion of the Jews of Christ’s time about major prophets dying on the same calendar day they were born or conceived.

The Jews had a lunar calendar, and Passover didn’t fall on the same day every year. Plus, the calendar sometimes added an extra month to keep it from diverging too much from the seasons.

On the other hand, the Romans had a solar calendar (created in 45 B.C. under Julius Caesar), so it was no easy thing for the Christians of the late-second and early-third centuries to figure out for sure the year of Christ’s death and what date on that year’s Jewish calendar would correspond to what date on the Roman calendar.

As such, the real issue was trying to determine the dates for Good Friday and Easter, Tighe says. Having settled on March 25 as the date of Christ’s death, the Jewish concept of “integral age” would have dictated belief that he was also conceived on March 25. Today, Catholics celebrate that day as the Feast of the Annunciation.

If Jesus was conceived on March 25, that means he would have been born nine months later — Dec. 25.

According to Tighe, the Greeks — working with a different calendar of their own until about 300 A.D. — settled on April 6 as the date of Christ’s death. Apply the same “integral age” process, and you get Jan. 6 as the date of his birth. The Western Church wound up accepting both, he says, one as the date for Christmas and the other as Epiphany.

Whether Jesus was actually born on either date is probably unknowable. Tighe acknowledges that the practice of actually celebrating Christ’s birth seems to have only started somewhat later — first in the West and then in the East. But pagan connections, he says, are most likely a stretch.