How Is Easter Determined?
Easter falls on the first Sunday after the Full Moon date, based on mathematical calculations, that falls on or after March 21. If the Full Moon is on a Sunday, Easter is celebrated on the following Sunday.
Although Easter is liturgically related to the beginning of spring in the Northern Hemisphere (March equinox) and the Full Moon, its date is not based on the actual astronomical date of either event.
March 21 is the church's date of the March equinox, regardless of the time zone, while the actual date of the equinox varies between March 19 and March 22, and the date depends on the time zone.
The date of the Paschal Full Moon, used to determine the date of Easter, is based on mathematical approximations following a 19-year cycle called the Metonic cycle.
Both dates may coincide with the dates of the astronomical events, but in some years, they don't.
Astronomical vs. Ecclesiastical Dates
In years in which the church's March equinox and Paschal Full Moon dates do not coincide with the astronomical dates of these events, confusion about the date of Easter can arise. In 2019, for example, the March equinox in the Western Hemisphere happened on Wednesday, March 20, while the first Full Moon in spring was on Thursday, March 21 in many time zones. If the church followed the timing of these astronomical events, Easter would have been celebrated on March 24, the Sunday after the Full Moon on March 21.
However, the Full Moon date in March specified by the church's lunar calendar, also called the ecclesiastical Full Moon, was March 20, 2019—one day before the ecclesiastical date of the March equinox, March 21. For that reason, the Easter date 2019 is based on the next ecclesiastical Full Moon, which is on April 18. This is why Easter 2019 was on April 21.
Earliest and Latest Easter Dates
According to the Metonic cycle, the Paschal Full Moon falls on a recurring sequence of 19 dates ranging from March 21 to April 18. Since Easter happens on the Sunday following the Paschal Full Moon, it can fall on any date between March 22 and April 25 (years 1753-2400).
Holidays Related to Easter
The dates of many Christian feasts are based on the date of Easter Sunday. For example:
Ash Wednesday (46 days before Easter)
Palm Sunday (1 week before Easter)
Maundy Thursday (3 days before Easter)
Good Friday (2 days before Easter)
Easter Saturday (1 day before Easter)
Easter Monday (1 day after Easter)
Ascension Day (39 days after Easter)
Whit Sunday / Pentecost (49 days after Easter)
Whit Monday (50 days after Easter)
Trinity Sunday (56 days after Easter)
Corpus Christi (60 days after Easter)
Why Is Easter Celebrated in Spring?
According to the Bible, Jesus Christ's death and resurrection occurred at the time of the Jewish Passover, which was celebrated on the first Full Moon following the vernal equinox. This soon led to Christians celebrating Easter on different dates. At the end of the 2nd century, some churches celebrated Easter on the day of the Passover, while others celebrated it on the following Sunday.
In 325 CE, the Council of Nicaea established that Easter would be held on the first Sunday after the first Full Moon occurring on or after the vernal equinox. From that point forward, the Easter date depended on the ecclesiastical approximation of March 21 for the vernal equinox.
Orthodox Easter
Not all Christian churches observe Easter according to the Gregorian calendar. Most Orthodox Christians still observe Easter under the Julian calendar.
The Gregorian calendar was created because the Julian calendar was slightly too long. With the Julian calendar, the equinox date moved towards the earlier dates of March and further away from Easter Sunday. The introduction of the Gregorian calendar allowed for a realignment with the equinox.
Like the Easter date based on the Gregorian calendar, Orthodox Easter falls on a Sunday between March 22 and April 25 in the Julian calendar (years 1753-2400). In the Gregorian calendar, this corresponds to April 3 to May 10.
In 2017, Easter fell on the same day (April 16) in both calendars when the Julian date was converted to the Gregorian date. The next time the two Easter dates coincide will be in 2025 (April 20).
Proposed Easter Date Reforms
There have been a number of suggested reforms for the Easter date. For example, in 1997, the World Council of Churches proposed a reform of the Easter calculation to replace an equation-based method of calculating Easter with direct astronomical observation.
This would have solved the Easter date difference between churches that observe the Gregorian calendar and those that observe the Julian calendar. The reform was proposed to be implemented in 2001, but it is not yet adopted.
Another example of a proposed reform occurred in the United Kingdom, where the Easter Act 1928 was established to allow the Easter date to be fixed as the first Sunday after the second Saturday in April. However, this law was not implemented, although it remains on the UK Statute Law Database.
