This Christmas, whether you celebrate the arrival of Santa Claus, the birth of Jesus Christ or the chance to eat Chinese food and see any movie you want, spare a thought for Pope Liberius. He’s the man responsible for setting its date as Dec. 25.

And his probable reasons for doing so should give pause to the holiday’s most devout champions and its shrillest critics.

Ever wonder why some Christian holidays, such as Easter, Ash Wednesday and Ascension Day, are “movable feasts,” changing around the calendar from year to year, while others, such as Christmas and Epiphany, do not?

The reason is that the movable feasts are much older, celebrated from the earliest days of the church. They were based loosely on the lunar Hebrew calendar. Passover is celebrated on the day of the first full moon of spring, and the Last Supper was a Seder. So early Christians marked Good Friday (the day of the crucifixion) and Easter (the Resurrection) soon after Passover. (In 325 A.D., Easter was fixed on the first Sunday after the first full moon of spring.)

Christmas was only added to the Christian calendar in the fourth century, and, frankly, it was added as a marketing ploy.

We don’t know the actual date of the birth of Jesus, but St. Luke (one of the two Gospel writers who mention the nativity) noted that at the time the “shepherds were abiding in the field and keeping watch over their flocks by night.” That would imply a date in the spring or summer, when the herds were up in the hills, not in the winter, when the animals were kept in corrals.

Saturnalia’s Role

So why did Pope Liberius decide in 354 to celebrate Christmas on Dec. 25? Well, in the absence of a fourth-century tell-all by one of Bob Woodward’s forerunners, we can’t really be sure. But we do know two things: First, when Liberius took office, the church was beset by a heresy known as Arianism, which taught that Jesus Christ was not truly divine but rather a created being. Second, Dec. 25 fell right at the end of the Saturnalia, the ancient Roman solstice festival, celebrated from Dec. 17 to Dec. 24. In the fourth century, after the Emperor Constantine (who ruled from 306 to 337) converted to Christianity, the new religion spread very rapidly. But the pagan Saturnalia was a very popular festival. It was marked with parties, gift-giving, decorating the houses with evergreens (sound familiar?), and a lot of drinking and sex.

By setting the date of Christmas as Dec. 25, Pope Liberius was both affirming the divine birth of Christ against heretics and telling would-be Christians that, in effect, “you can convert to Christianity and still enjoy the Saturnalia.”

Medieval Christmas, like its pagan ancestor, was a raucous, usually drunken affair. Many dioceses had a “feast of fools,” in which one of the lower clergy was temporarily installed as a “bishop of fools” and fun was made of church ceremonial. In France, churches held a “fete de l’ane,” in honor of the donkey who brought Mary to Bethlehem. Outside the church there was community-wide feasting, drinking and, it’s safe to assume, sex.

But with the Reformation, things changed. Many Protestant sects abolished Christmas as a medieval corruption. It was banned in Scotland in 1563 and in England when the Puritans defeated and executed Charles I. Puritan New England also did not celebrate Christmas. Although the holiday was revived after the Restoration, it never again recovered its raucous medieval ways. Instead, it became a much more family-oriented occasion.

Modern Myth

In the early 19th century, a group of New York writers, including John Pintard, Washington Irving and especially Clement Clarke Moore, established the modern American secular Christmas myth centered on children, with Santa Claus carrying presents in his reindeer-powered sleigh and popping down chimneys to deliver them. Poinsettias, which have become associated with Christmas because they bloom in December, were unknown until the American minister to Mexico, Joel Roberts Poinsett, brought them back from there in 1828. In the mid-19th century the cartoonist Thomas Nast created the modern image of Santa Claus as a jolly fat man with a white beard. (The red suit became standard only in the 1920s, thanks to Coca-Cola ads.) Christmas trees came to the English-speaking world about the same time, when the German Prince Albert -- some of whose ancestors were doubtless pagan tree worshippers -- married Queen Victoria.

Merchants, needless to say, pushed the idea of gift-giving, decorating their stores and having sales. As Christian sects that celebrated Christmas, such as Catholics and Anglicans, began to move into New England, where Christmas had been unknown in colonial times, the holiday was slowly revived there. Pressure from children (“The O’Reilly kids down the street are getting presents. Why aren’t we!?”) certainly helped. Churches that had previously not celebrated Christmas, such as the Presbyterians and Congregationalists, began to do so. Again, it was a marketing ploy, to keep congregants from migrating to churches that did celebrate Christmas.

The poet Henry Wadsworth Longfellow noted in 1856, “A transition state about Christmas here in New England. The old puritan feeling prevents it from being a cheerful, hearty holiday; though every year makes it more so.” More and more states, including those in New England, made Christmas a legal holiday and in 1870, President Ulysses S. Grant signed it into law as a federal holiday.

Today this secular Christmas is celebrated around the world even in countries, such as Japan and South Korea, where only a small percentage of the population is Christian. One of the most famous Christmas songs ever written, “White Christmas,” was written by Irving Berlin, who was Jewish.

So perhaps even First Amendment absolutists who picket stagings of “A Christmas Carol” and decry poinsettias as a Christian symbol should just relax. The Christmas of the Virgin Mary and the manger, the three kings and “Silent Night” is a Christian holy day celebrating the birth of Jesus. The Christmas of Santa Claus and Christmas trees, presents and parties and “Rudolph the Red-Nosed Reindeer” is a winter solstice festival celebrating the rebirth of the sun.

(John Steele Gordon is the author of numerous books, including “Hamilton’s Blessing: The Extraordinary Life and Times of Our National Debt.” The opinions expressed are his own.)

To contact the writer of this article: John Steele Gordon at jsg@johnsteelegordon.com.

To contact the editor responsible for this article: James Gibney at jgibney5@bloomberg.net.