At some point or another you've likely heard that Christmas, the observance of Jesus' birth, is celebrated on different dates by different groups of people. Here in the west churches all celebrate Christmas annually on December 25th, and it's a major holiday whether you're religious or not. But for many Eastern Orthodox, to give the most prominent example, Christmas is on January 7th. I'm here today to explain why that is, and it's all about calendars.
From the beginning, during and immediately following Jesus' life, until the 11th century there was only one Christian church (you know, the one based in Rome). It was an eventful 1000 years, to be sure, which included the fall of the western half of the Roman Empire, the beginning and spread of Islam and more. With much of the world crumbling around it the church remained whole and was the most important institution connecting the emerging kingdoms to the classical past. But I'm getting distracted (note to myself: several of these points would make fun posts of their own).
What's important here is the one world Christian church about 1000 AD used the Julian calendar. Proposed by Julius Caesar in 46 BC and adopted the following year, it adapted the Roman calendar to be 365 days (from 355) with a leap year every four years without exception. The average year length was then exactly 365.25 days. This calendar brought the calendar year into closer alignment with the solar year, even if it still wasn't perfect. It would be used by the western world (due to the Catholic church) until 1582, more than 1600 years.
The most consequential split in Christian history occurred in 1054 AD, almost 500 years before Martin Luther's 95 Theses. The Christian world was divided into two distinct groups, the Latin (Roman) west and the Greek east. This goes back before the unofficial separation of the Roman Empire into western and eastern halves. (This idea is another post I meant to write but didn't get to it, alas.) Basically, the coming of Rome to western Europe (i.e. Iberia, Gaul, Britain) was the beginning of their history. The pagans there recorded little themselves and we don't know a lot about them. On the other hand the east (i.e. the Greek world) had forged a strong identity over hundreds of years that wasn't going to be broken. So even though conquered by Rome, the Greeks maintained a great deal of their own culture. In 1054 the two groups within the church (Latin and Greek) broke up in what's called the East-West or Great Schism. The differences involved both power struggles (who was in charge in what city etc.) as well as theological fights. From 1054 there were then two churches, the Catholic church based in Rome and the Eastern Orthodox church based in Constantinople. The Schism continues to this day.
In 1582 the Catholic church, under Pope Gregory XIII, altered the calendar again a tiny bit. This included the creation of exceptions to the leap year rule, keeping a leap year every four years unless the year was a multiple of 100 but not a multiple of 400. So, by and large, it's a leap year every four years but it wasn't in 1700, 1800 or 1900. 1600 and 2000 were leap years. The next skipped leap year is 2100. I find it interesting we're in the middle of a 200 year stretch where the leap year will never be skipped, and wouldn't be surprising if you didn't know or remember this detail (I didn't.) The new calendar, the Gregorian calendar, made the average year 365.2425 days long, even closer to the true solar year of 365.2422 days. This calendar is now the primary calendar used worldwide, but adoption was often slow and in some countries was more recent than you might think, like Greece in 1923.
Just because the Gregorian calendar was universally adopted for secular purposes, including in eastern Europe, doesn't mean the Eastern Orthodox church has done the same for its religious calendar. In many places (Russia, Georgia, Macedonia, Montenegro, Serbia and Jerusalem), the church still uses the Julian calendar to set dates for its important celebrations. As of now December 25th on the Julian calendar is January 7th on the Gregorian calendar. So these Orthodox celebrate Christmas 13 days later than the rest of the Christian world. (Orthodox in other countries/areas have adapted their calendar to align with the Gregorian.) I think at this point the decision to do this is as much about religious tradition as it is about keeping cultural differences between east and west.
Maybe you're doing the math, and three skipped leap years over 400+ years does not equal 13 days. You're right. When the Gregorian calendar was adopted they also shifted the calendar 10 whole days to correct the extra leap days over about a 1200 year period. The day after Thursday October 4th 1582 was Friday October 15th. The calendars will continue to be off by 13 days until the next skipped leap year in 2100. That's one of the more fascinating pieces of all this to me, the two calendars are ever so slowly drifting farther apart, though the initial 10 day shift is still a majority of the difference.
To sum up and to put it simply, the reason some Orthodox have Christmas so much later is that they still use a different, older calendar, one that's not as accurate and has drifted pretty far from the solar calendar. They don't seem to care. The Julian calendar is another direct link to the Roman Empire too, in case you're thinking for something to really think about.
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