For most Americans, Christmas is the most important holiday of the year.

Yet that is largely an accident, stemming from a series of improbable changes spanning two millennia. A similar set of changes has affected the development of other seasonal holidays, like Hanukkah.

Early Christians did not celebrate the Nativity. Christianity had been around for more than 350 years before the church fathers in Rome decided to add that event to the Christian calendar. They did so in part because many Christians were arguing that Jesus had not been an actual human being but rather a divine spirit — a belief the church fathers considered heretical. What better way to convince Christians that Jesus was human than to commemorate his physical birth? The problem was that there was no evidence of when Jesus’s birth took place. (Neither Luke nor Matthew, the two gospel writers who included stories of Jesus’s Nativity in their narratives, had indicated the date, or even the season, of the event.)

The church fathers decided to place the new holiday in late December, virtually guaranteeing that it would be widely adopted because this was already a season of mid-winter revels, a holdover from pagan times. For the inhabitants of the Roman Empire, the holiday was called Saturnalia. This festival, which concluded on Dec. 23, was partly a holiday of lights that celebrated the winter solstice. But Saturn was the god of agricultural abundance, so his festival also marked the bounty of the completed harvest. Finally, the Saturnalia was a time of role reversals and seasonal license. Everyone took time off from ordinary labor. Slaves were granted temporary freedom and were treated by their masters to lavish banquets. The holiday was observed with feasting, drinking, gambling and sexual abandon.

As the church fathers hoped, Christmas became an important holiday. But by placing it at such a time, they all but gave up the ability to define it as a purely religious one: Christmas was not easy to Christianize. In fact, the festivities that for centuries would mark its celebration resembled those of Saturnalia and other mid-winter rituals. In England, bands of young men roamed from house to house, singing as they begged for alcohol and money. Later, in the American South, slaves were occasionally granted temporary freedom, encouraged to get drunk and, yes, treated by their masters to lavish banquets. All that helps explain why, as a consequence of the Protestant Reformation of the 1500s, Puritans in both Old and New England tried to suppress Christmas as a vestige of paganism. From 1659 to 1682 it was actually illegal to celebrate the holiday in Massachusetts.

In the 1800s, Christmas was transformed into the familiar domestic and child-centered ritual it remains to this day, centered on the magical figure of Santa Claus. The transformation was both marked and abetted by the 1823 publication of Clement Clarke Moore’s poem “A Visit From St. Nicholas” and, two decades later, by the proliferation of Christmas trees in the United States. At the same time, Christmas also became a commercial holiday. Even before 1830, shopkeepers were using Santa Claus to tout their wares, and the first Christmas-tree vendors appeared in the streets during the 1840s. Indeed, from the very beginning, the family-focused Christmas and the commercial Christmas have worked in tandem to reinforce each other.

Vestiges of the older mid-winter traditions remain. Christmas lights, wassail songs and mistletoe hark back to those traditions. (So, in a way, does the occasional presence of the “naughty” Santa.) Then there are the office Christmas parties, with their whiff of alcohol and flirtation. And what about New Year’s Eve, to which the boisterous old-time Christmas revels have been mostly relegated?

The newer as well as the older Christmas practices involve powerful traditions of excess, when people flout the rules of ordinary behavior with impunity. In the old days, that violation often involved great feasting and drinking, bawdy revels and the reversal of social roles. Nowadays, it involves excessive spending — for presents that are almost by definition luxuries rather than necessities.

Other, competing mid-winter traditions mirror those of the Christian holiday, and those traditions, too, have been changed by a series of historical accidents. Take the Jewish holiday of Hanukkah, the Jewish Festival of Lights.

Hanukkah originated nearly six centuries before Christmas as the celebration of a Hebrew military victory, the liberation of Jerusalem from its Macedonian-Greek occupiers. The story of the miracle of lights — the meager quantity of oil that burned for eight days to cleanse the profaned temple — emerged as the central meaning of the holiday only 400 years afterward, after a subsequent (and unsuccessful) revolt against the Romans, probably because by then the Jewish leadership did not wish to draw attention to that successful earlier revolt against another occupying nation.

As Hanukkah was transformed into a festival of lights, commemorated by the nine-candled menorah, it, too, came to take on seasonal associations. The Jewish Talmud itself hints at linking Hanukkah not only to the winter solstice but also — like Christmas — to the completion of the harvest. (Not surprisingly, in the modern state of Israel the original military victory has reemerged as a central element of the Hanukkah story.)

In recent times, Hanukkah, too, has largely become a child’s holiday. Many Jewish parents give their children seasonal presents as abundant — and expensive — as those received by their Christian neighbors.

And with Hanukkah as with Christmas, a vestige remains of older mid-winter festivals. This is the dreidel, a four-sided top that resembles the familiar six-sided dice and is used in similar fashion to determine how much money (or Hanukkah “gelt”) the player receives — or owes. Thus Hanukkah, originating as the celebration of a military victory, now incorporates a host of other rituals: the commemoration of a divine miracle, a seasonal celebration of light and harvest, a focus on children and even a hint of mid-winter revelry.

Over the centuries, through all those historical accidents, Hanukkah and Christmas have come to look a lot like each other.

Stephen Nissenbaum is the author of
The Battle for Christmas.”