By Dr John Newton
The different dates used by the various Christian churches for the celebration of Easter and Christmas stem from a fascinating history about attempts to align the calendar accurately with the seasons, and the key mysteries of the Christian faith.
Every year, when Christmas and Easter come around, there is usually a flurry of pieces in the media saying that the world’s Eastern Orthodox Christians — who number between 225 and 300 million — are celebrating Christmas on 6 January according to the ancient Julian calendar, as opposed to 25 December in the more familiar Gregorian calendar. This, however, is a gross oversimplification: only just under half of the world’s Eastern Orthodox follow the Julian calendar.
Eastern Orthodoxy is the world’s second-largest Christian communion, sharing much of its early history with the Roman Catholic Church, which it parted ways with in the 11th century. Comprising 15 self-governing churches, and several daughter churches too, Eastern Orthodox Christians live in more than 60 countries and account for about 12 per cent of all Christians.
There is a similar divergence on the date of Easter, which for the Orthodox can fall between 4 April and 8 May; but for others can fall between March 22 and April 25. Rarely, the dates align and Easter is celebrated simultaneously.
Julian calendar
The Roman Empire used a lunar calendar, which was based on the phases of the moon. In theory, the calendar was kept aligned with the seasons by the introduction of a leap month every few years.
This was determined by the Pontifex maximus, the high priest of Rome’s College of Pontiffs, but there was dissatisfaction with this system as the introduction of the leap month could be a subjective process — and since the lunar year determined the Pontifex maximus’s time in office, it was not unknown for an unscrupulous high priest to add a leap month for personal gain.
Julius Caesar commissioned the astronomer Sosigenes to design a solar calendar, based on the earth taking 365¼ days to orbit the sun. This calendar introduced the 12 months familiar to us, each having 30 or 31 days except February, which has 28 days or 29 in leap years, which occurred automatically every fourth year. The calendar came into effect in 45BC.
When Christmas began to be publicly celebrated in the fourth century, it was assigned to 25 December in the Julian calendar, one of two dates for Jesus’s birth that circulated in the early church. The earliest manuscript support we have for this date comes from the Chronicon written c.235 by the Christian theologian Hippolytus of Rome.
The next century, in the year 325, the churches held the first ecumenical council at Nicaea. To unify different practices the council decreed that Easter should be observed on the first Sunday following the first full moon after the Spring Equinox.
In 525 the monk Dionysius Exiguus developed a method to predict the date of Easter according to the rules laid down by Nicaea. This gained widespread acceptance and is often referred to as the Julian method for calculating Easter.
However, Sosigenes — Caesar’s astronomer — had overestimated the length of the year by more than 11 minutes, meaning that by the Middle Ages the calendar was no longer aligned to the seasons.
Gregorian calendar
The Council of Trent (1545-63) called for a revision of the calendar, as the date of Easter was drifting further away from what Nicaea had envisioned.
Pope Gregory XIII oversaw the reform of the calendar, changing the rules for when a leap year occurs: all years divisible by four are leap years, but not centurial (end-of-century) years that cannot be divided by 400. He also removed 10 days from the calendar, so the Spring Equinox would again align with 21 March as it did at the time of the First Council of Nicaea. The reforms also revised the method used for calculating Easter, which attempted to predict the date more precisely according to the rules laid down at Nicaea.
This all went into effect in 1582 in Catholic countries, where Thursday 4 October was followed by Friday 15 October. That meant those following the Gregorian calendar would celebrate Christmas and sometimes Easter on different days to those retaining the Julian system.
Despite being more accurate, the Gregorian calendar was adopted only cautiously outside of the Catholic world. Other countries in Europe introduced it gradually, but Great Britain did not adopt it until September1752. Greece waited until 1923.
However, many churches still used the Julian calendar for religious dates. For example, the Ukrainian Greek Catholic Church continued using the Julian method even though it entered into full communion with the Pope at the end of the 16th century. It switched to Gregorian in 2023, during the war between Ukraine and Russia, probably to create distance from the Russian Orthodox, which uses the Julian calendar.
Revised Julian calendar
In the early 1920s, the Serbian mathematician and astronomer Milutin Milanković proposed revising the Julian system to introduce a more accurate calendar and overcome the discrepancy between the Julian and Gregorian. Milanković did this by introducing a new rule for determining leap years: that all years divisible by four are leap years, centurial years are leap years only if there is a remainder of 200 or 600 when they are divided by 900.
A joint committee composed of members of both the Greek government and the Greek Orthodox Church considered the proposal at a meeting on calendrical reform. The Greek state eventually decided to adopt the Gregorian calendar. In 1923 the Greek Orthodox Church went on to consider the question in conjunction with other Eastern Orthodox churches and adopted the revised Julian scheme.
Today most Eastern Orthodox churches use the revised Julian calendar — which will be identical to the Gregorian calendar until 2800 — with only six Orthodox churches retaining the old Julian calendar: Georgian, Macedonian, Polish, Russian, Serbian, and the Greek Orthodox Patriarchate of Jerusalem. However, since the Russian Orthodox Church is so numerous, this means just under half of all Eastern Orthodox Christians still follow the Julian calendar.
Several Eastern Catholic churches also use the revised Julian calendar: the Bulgarian Greek-Catholic, Romanian Greek-Catholic, and Greek Byzantine-Catholic Churches. It is also used by the Melkite Greek-Catholic Church in some countries in the Middle East. The Melkites generally employ the calendar used by the majority of churches in a particular country.
A new astronomical method for calculating Easter was also proposed by Milanković, but no church accepted it. Dionysius Exiguus’s method is still used for Easter by the Eastern Orthodox, except for the Finnish Orthodox Church which uses the Gregorian method.
Oriental Orthodox churches
It is worth mentioning the calendars employed by the Oriental Orthodox churches, the family of churches that broke from communion with the rest of mainstream Christianity following the Council of Chalcedon in 451. The split was caused by theological misunderstandings over the divinity and humanity of Jesus — both sides believed him to be fully human and fully divine, but expressed this mystery in different ways.
The Coptic Orthodox follow the old Coptic calendar that comprises 13 months: 12 months of 30 days followed by a month of five days, which is extended to six days in a Coptic leap year. Leap years come automatically every four years, as in the Julian calendar. The Ethiopian and Eritrean Orthodox Tewahedo Churches essentially use the same calendar. Christmas is celebrated at the same time as 25 December in the Julian calendar, which is 7 January in the Gregorian system.
The Armenian Apostolic Orthodox Church uses the Gregorian calendar, but celebrates the Feast of the Nativity and Theophany of the Lord on 6 January. This is the same date as Epiphany in the Gregorian calendar, which marks the same events in Jesus’s life as the Theophany, the visitation of the Magi, Jesus’s baptism in the Jordan, and his transformation of water into wine at the wedding in Cana.
The Syriac Orthodox Church follows the Melkite practice of adopting either the Julian or Gregorian calendar, depending on which is used by most churches in that country. However, its daughter church, the Malankara Orthodox Syrian Church, adheres strictly to the Julian.
All Oriental Orthodox churches use Dionysius Exiguus’s method to determine the date of Easter.
Dr John Newton is communications and research manager for action for Aid to the Church in Need, UK
