Photograph by David Mdzinarishvili, Reuters
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Children participate in a Christmas procession in Tbilisi, Georgia on January 7, 2016. Orthodox believers observe Christmas according to the Julian—rather than Gregorian—calendar.

Photograph by David Mdzinarishvili, Reuters
HistoryExplainer

Why some people celebrate Christmas in January

Thanks to a decision made more than 400 years ago, not all Christians observe the holiday on December 25.

Christmas is over, so why are 12 percent of the world’s Christians waiting until January to celebrate? The answer lies in many Orthodox churches’ decision to adhere to a nearly 2,000-year-old calendar that differs from the one used by most of the world today.

The origins of the split over when to officially recognize the birth of Jesus Christ stretch back all the way to A.D. 325, when a group of Christian bishops convened the religion’s first ecumenical conference. One of the First Council of Nicaea’s most important agenda items was to standardize the date of the church’s most important holiday, Easter. To do so, they decided to base it on the Julian calendar, a solar calendar which Roman ruler Julius Caesar had adopted in 46 B.C. on the advice of Egyptian astronomer Sosigenes in an attempt to clean up Rome’s own messy lunar calendar.

But Sosigenes’ calculations had an issue of their own: They overestimated the length of the solar year by about 11 minutes. As a result, the calendar and the solar year became increasingly out of sync as the centuries progressed.

By 1582, the dates of important Christian holidays had drifted so much that Pope Gregory XIII was concerned. He convened another group of astronomers and proposed a new calendar, known as the Gregorian calendar.

The new calendar solved a number of tricky issues that had accumulated over the years, and the majority of the Christian world adopted it. But the Orthodox Church, which had split into its own arm of Christianity during the Great Schism of 1054 after centuries of increasing conflict, objected to the change. Following Pope Gregory’s course correction would mean accepting an occasional overlap between Passover and Easter—a move that went against holy texts of Orthodox Christianity. So the Orthodox Church rejected the Gregorian calendar and continued to rely on the Julian calendar.

It stayed that way for centuries, and the calendar drift continued for Orthodox churches. By 1923, there was a 13-day difference between the two calendars, putting Orthodox Christmas 13 days after December 25.

That explains the existence of two Christmases, but the calendar crisis continued for Orthodox churches. In May 1923, a group of Orthodox leaders met to hash out the issue. Held in Constantinople, the Pan-Orthodox Congress brought together delegations from the churches of Constantinople, Cyprus, Greece, Romania, Russia, and Serbia.

Discussion was heated: Historian Aram Sarkisian writes that the Church of Russia had been pressured to adopt the Gregorian calendar by the Bolsheviks, who abandoned the Julian calendar shortly after the Russian Revolution began. Revising the calendar wasn’t just a matter of religion: To churches whose existence was threatened under the rule of Communism, a calendar adjustment was a matter of survival.

At the conference, Serbian scientist Milutin Milanković proposed a solution: a new version of the Julian calendar that shares its dates with the Gregorian calendar, though it doesn’t share every leap year. Known as the revised Julian calendar, it was adopted by several Orthodox churches after the council, including the churches of Greece, Cyprus, and Romania. Those churches now celebrate Christmas on December 25. Other Orthodox churches, like those of Russia and Egypt, refused. And still others, like Poland, adopted Milanković’s calendar, then dropped it later.

Today, Orthodox Christianity is the third largest Christian denomination with an estimated 260 million followers. Will calendar confusion eventually lead them to another change? Perhaps, but until then Christmas cheer will have to wait until January for many Orthodox Christians.