Did you ever wonder why Christmas falls on Dec. 25? After all, the Bible provides no birth date for Jesus.

Or does it? There’s a possible clue in Luke 2:8, which says that when Jesus was born shepherds were “out in the field, keeping watch over their flock by night.” Some figure a wintertime birth was unlikely because sheep would have been kept in a corral, not out in the countryside.

However, others report winter sheepherding in open fields in the Holy Land, meaning we can’t fix the season of the year, much less the date, from Scripture.

Why, then, Dec. 25?

The most popular explanation is probably wrong, asserts Andrew McGowan, a historian at Episcopal Divinity School in Massachusetts, in the December issue of Bible Review magazine.

That popular view notes that in A.D. 274, the Roman Emperor Aurelian established Dec. 25 to celebrate the advent of Sol Invictus (the Unconquered Sun) near the point when days start getting longer. (The year’s least sunny point, or winter solstice, comes around Dec. 21 by modern reckoning.)

Therefore, the theory goes, the Christians fixed their own great birthday observance to borrow from, and challenge, the pagan holiday.

Sounds plausible, but McGowan is convinced the choice actually stemmed from Judaism, not pagan competition.

For one thing, ancient writings give no support for the theory. Also, there’s no evidence that early Christians celebrated Jesus’ birthday at all. Some second-century Christian authors mocked the pagans for holding such birthday festivals.

Nor is there any proof that Christians were adapting pagan festivals in the century during which Dec. 25 became established. Quite the contrary, McGowan says.

He finds the connection highly unlikely because up to the last violent attacks on Christianity under the Roman Emperor Diocletian (A.D. 303 and 312), “the persecuted Christian minority was greatly concerned with distancing itself from the larger, public pagan religious observances.”

THE NATIVITY AS DEPICTED in a print by French artist Paul Gustave Dore to illustrate his 1865 book, The

Adaptation of pagan dates and themes didn’t begin until later in the fourth century, after Christianity won toleration from the empire, he says. The high point came under Pope Gregory “the Great” (reigning A.D. 590-604).

The first claim we know about concerning Christians deliberately setting Christmas to coincide with the pagan date was written in the 12th century, in a marginal note to a Bible commentary. The paganism theory became fashionable among scholars in the 18th and 19th centuries when the new field of comparative religion arose, McGowan says.

The earliest surviving mention of Dec. 25 for Christmas occurs in a Roman almanac that Bible chronology expert Jack Finegan dated at A.D. 354, with a probable earlier edition produced in A.D. 336.

In A.D. 386, a sermon by Syria’s John Chrysostom said Dec. 25 had been observed there for less than a decade, but far longer among western Europeans.

Finegan said that Christians writing in earlier times listed all sorts of supposed dates for the Nativity, including (by modern calculation) Jan. 6 or 10, April 18 or 19, May 20 or 21, and Nov. 18.

McGowan favors the Jewish explanation, developed by Thomas Talley in “Origins of the Liturgical Year” (1991).

The New Testament records that Jesus was crucified during the spring Jewish Passover. And Jewish tradition held that great things were expected to occur “again and again, at the same time of the year,” McGowan writes.

By this sort of ancient thinking, if Jesus died in spring he must have been conceived in that same season, with the birth date coming nine months later.

Eventually, the traditional liturgical calendar marked the Annunciation (Gabriel’s revelation to Mary of her holy pregnancy) on March 25, followed in nine months by Christmas on Dec. 25 — or April 7 and Jan. 6 in the calendar of the Armenian Church.

(Another calendar complication: Many tradition-minded Orthodox churches in eastern Europe reject the common Gregorian calendar set by the papacy and retain ancient Rome’s Julian calendar; with the 13-day difference, their Christmas isn’t Dec. 25 but Jan. 7.)