If you’re not an Orthodox Christian, you probably don’t understand why January 7 is Orthodox Christmas. The reason is found, at least partly, in astronomy.
In 46 BC, Julius Caesar set up the calendar system which the Roman Empire would use for centuries and European countries would inherit. The Julian calendar added an extra day every four years to make up for the regular calendar having 365 days in a year but the earth’s orbit actually being about a quarter day longer. That kept the spring equinox at about March 21 every year–for some time.
Adding an extra day every four years is actually an overcorrection. It assumes Earth’s orbit is exactly 365 days, 6 hours long. But it’s really 365 days, 5 hours, 48 minutes, and 46 seconds. Simply adding that extra leap year day every four years without fail adds up to a discrepancy of a full day after 128 years. The equinox happens a day earlier.
That wasn’t a big deal at first. An equinox on March 20 instead of March 21 wasn’t something most people noticed. By the time the equinox crept back to March 19, the people who remembered it having been on March 21 were all gone. But after about 16 centuries, the equinox was happening about March 11. For the Roman Catholic Church which still used the old calendar for determining when to celebrate religious festivals, it caused problems.
The main problem was that the date of Easter, which happens after the equinox, was also creeping earlier and earlier. The Church’s leaders felt it was wrong to celebrate Easter too early. So Pope Gregory XIII ordered an adjustment. He clipped ten days from the calendar in 1582. The day after October 4 was October 15. Another way of looking at the new Gregorian calendar was to say it added ten days to whatever the date was under the Julian calendar. Equinoxes in the years following using the new calendar were returned to about March 21 when the old calendar was only at March 11.
Gregory’s order also removed three leap year days every four centuries. If a year is divisible by 100, it is also divisible by 4. However, it is not a leap year in Gregory’s calendar unless it is also divisible by 400. So 1600 and 2000 were leap years with 366 days. But 1700, 1800, and 1900 had just 365 days. That’s not as close to the ideal situation of removing one leap year every 128 years, but it won’t be until 4909 when the Gregorian calendar is again a full day off when it comes to keeping the March equinox on or about the 21st.
In 1582, countries where Protestant and Orthodox Christianity were dominant wanted nothing to do with the Catholic Church’s calendar reform. They continued to use the Julian calendar they were familiar with. Sometimes, leaders in the other faiths denounced the Gregorian calendar as part of a Vatican attempt to regain control in Protestant and Orthodox lands. But after more than a century, political and practical considerations merged as the equinox crept even earlier on the Julian calendar and international business relations were increasingly inconvenienced by having to keep track of two out-of-step but nearly identical calendars.
The other countries in Europe began converting to the Gregorian calendar in 1700. The British Empire, including the colonies in North America, switched in 1752. Britain was among the last Protestant countries to make the change. Most churches in those countries accepted the Catholic way of deciding when to observe Easter.
But Orthodox Christians and the countries where they were the majority didn’t feel as much pressure to change. They continued using the Julian calendar into the early 1900s. By then, it lagged its rival by 13 days. Deciding it was time to adopt the calendar that even non-Christian parts of the world had begun using, their governments began using the Gregorian calendar. Greece was the last of them in 1923. However, unlike Catholics and Protestants, Orthodox Christians didn’t completely set the Julian calendar aside.
As far as today’s Orthodox Christians are concerned, when it’s December 25 in the United States and Italy, it’s also December 25 in Russia and Greece. But they usually prefer not to celebrate Christmas on that day. They wait 13 more days until it’s December 25 on the Julian calendar. That Gregorian calendar date is January 7.