Orthodox Calendar: Julian vs Gregorian Explained

Use the tool above to convert any date between the Julian and Gregorian calendars, or to look up this year's Orthodox feast days in both. Below, we explain why Orthodox Christians use two different calendars at once, why Christmas falls on two different days depending on which church you attend, and why Pascha is the one date everyone agrees on regardless of which calendar their parish follows.

Two calendars, one Church

Some Orthodox Christians celebrate Christmas on December 25th. Others celebrate it on January 7th. It's the same feast — they're simply using different calendars. Neither group is wrong, and the difference doesn't divide the Church in any way that affects communion, sacraments, or apostolic succession. An Orthodox Christian from a parish using one calendar can receive the Eucharist at a parish using the other without any theological obstacle. What looks at first like a confusing inconsistency is really just two administrative systems inside one Church.

Why does the 13-day gap exist?

The Julian calendar, introduced by Julius Caesar in 46 BC, was the standard calendar of the Christian world for over 1,600 years. Its only flaw is a small one: it adds a leap day every four years without exception, which is slightly too generous. The actual solar year is a little shorter than 365.25 days, so the Julian calendar drifts about one extra day every 128 years compared to the actual seasons.

By the 16th century, this drift had become impossible to ignore — the calendar date of the spring equinox had wandered noticeably from the actual astronomical equinox. In 1582, Pope Gregory XIII introduced a corrected calendar, the Gregorian calendar, which refined the leap year rule (skipping it in century years not divisible by 400) and removed the accumulated drift in one step. Catholic Europe adopted it quickly. Protestant countries followed more slowly, some over a century later. Eastern Orthodox countries took even longer: Russia didn't adopt the Gregorian calendar civilly until 1918, and Greece held out until 1923.

The gap between the two calendars isn't fixed — it grows roughly one day every century, because that's the rate at which the Julian calendar's small inaccuracy accumulates. Right now, in the 21st century, the gap is 13 days. It will become 14 days starting in March 2100.

Why didn't the Orthodox Church just switch?

The Orthodox Church didn't reject the Gregorian calendar's astronomy — the math is correct, and nobody disputes that. The objection was about something else entirely: the date of Pascha. The First Ecumenical Council at Nicaea in 325 had settled, as a matter of Church-wide unity, how Pascha should be calculated, tying it to the Julian calendar's reckoning of the spring equinox. Adopting the Gregorian calendar's own method for calculating Easter would have occasionally placed Pascha on the same day as, or even before, the Jewish Passover — a result the ancient canons explicitly forbid. Rather than risk that, the Orthodox Church kept the old calculation for Pascha while debating what to do about everything else.

The 1923 compromise — and why it only half-solved things

In May 1923, a congress of Orthodox churches in Constantinople proposed a compromise: the Revised Julian calendar, designed by the Serbian scientist Milutin Milanković. It uses a more accurate leap-year rule than the original Julian calendar, and it's been mathematically identical to the Gregorian calendar since 1900 and will remain so until the year 2800. The idea was to use this corrected calendar for fixed feasts — Christmas, Theophany, the Annunciation, and so on — while still calculating Pascha and the moveable feasts that depend on it (Palm Sunday, Pentecost, the start of Great Lent) using the old Julian Paschalion, exactly as before.

Some churches adopted this compromise. Others didn't. Today:

• Churches using the Revised Julian calendar for fixed feasts — Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Bulgaria, and most of the Orthodox Church in America — celebrate Christmas on December 25th, the same day as Western Christians.
• Churches still using the old Julian calendar for everything — Jerusalem, Russia, Serbia, Georgia, Poland, and Mount Athos — celebrate Christmas on December 25th by their own calendar, which currently falls on January 7th by the Gregorian count.

And critically, every single Orthodox Church — without exception except Finland and Estonia — still calculates Pascha using the old Julian Paschalion. This is the detail that resolves most of the apparent confusion: the fixed-feast calendar split happened in 1923, but the Paschalion never did. Whether you're at an Antiochian parish in Beirut or Boston, a Serbian parish in Belgrade, or a Greek parish in Athens, Orthodox Pascha falls on the same day for everyone, every year — and it's frequently a different day from Western Easter, sometimes by as much as five weeks, because the two methods of calculating it are genuinely different at the mathematical level, not just offset by 13 days.

So which calendar does my parish use?

It depends entirely on jurisdiction, not on what's theologically "more correct." If you're visiting an unfamiliar parish, it's worth checking in advance which calendar they follow, simply so you don't miss a feast day or show up expecting a fast to have ended when it hasn't. A small number of communities, known as Old Calendarists, treat the calendar question as a test of Orthodox identity and have broken communion with their mother churches over it — but this is very much the exception, not the mainstream position. For the overwhelming majority of Orthodox Christians, the calendar difference is simply a practical fact of life, not a theological dividing line.

Frequently asked questions

Is the Julian calendar "wrong" and the Gregorian calendar "right"?

Astronomically, yes, the Gregorian calendar tracks the solar year more precisely. But the Orthodox Church's continued use of the Julian calendar for the Paschalion isn't a scientific position, it's a canonical one — rooted in preserving the method of calculating Pascha established at the First Ecumenical Council, not in any claim about whose calendar measures the year better.

Will Orthodox Christmas ever align permanently with December 25th everywhere?

Churches using the Revised Julian calendar already celebrate fixed feasts like Christmas on December 25th, identical to the Western date, and this alignment will hold until the year 2800. Churches still on the old Julian calendar would need to make their own decision to adopt the revised version — there's no indication this is imminent for Russia, Serbia, or Jerusalem.

Why does Orthodox Easter sometimes match Western Easter and sometimes differ by weeks?

Both traditions calculate Easter based on the first Sunday after the first full moon following the spring equinox, but they use different equinox dates and different lunar tables to do it. Some years the two calculations land on the same Sunday by coincidence. Most years they don't, and the Orthodox date falls one to five weeks after the Western one, since Orthodox Pascha can never precede Passover according to the ancient canons.

Does the calendar difference affect Communion between Orthodox Christians?

No. Orthodox Christians remain in full communion with one another regardless of which calendar their parish follows. The split is purely administrative, not sacramental or doctrinal.

To see how this plays out across the full liturgical year, our guide to the great Orthodox feasts walks through each one in turn, and our guide to Holy Week day by day traces the days leading up to Pascha itself.