Easter Sunday falls on a different Sunday every year. Using the 1928 Book of Common Prayer of the Episcopal Church, Easter falls on the first Sunday, between March 21st and April 19th, after the Paschal full Moon that occurs on or after the spring equinox. If the full moon lands on a Sunday, then Easter is the following Sunday
The date is fixed in accordance with an ancient ecclesiastical computation, and does not always correspond to the astronomical equinox. The Metonic cycle of 19 years is one in which the phases of the Moon repeat exactly. It is thus possible to have a 19-year cycle for the dates of full or new Moon. In the Julian calendar this 19-year cycle can be fairly easily translated into a date for Easter.
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.
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.
In today’s Gregorian calendar the calculation is complicated by the definition of which century years are leap years. These leap years mess up the simple Metonic cycle by altering the number of days in different periods of 19 years.
On page lii and liii of the 1928 BCP, you can find the calculation for determining when Easter Sunday is. It requires the following steps:
- Find the Golden number or “Prime”
- Find the Sunday (Dominical) Letter
- Determine Easter Sunday on Calendar with item # 1,2
The Golden number or Prime can be found by adding 1 to the current year and divide by 19, the remainder if any, is the golden number or Prime. If no remainder exists, the golden number is 19
The Sunday (Dominical) letter is found by finding the next preceding 100th year which currently is 2000. The remainder is 23 (if calculating for the year of our lord 2023). 23 intersects with the “Hundreds of Years” table and the “Years in excess of Hundreds of Years” table at “A”. Therefore the Sunday letter ia A.
For example, for the year 2023, the golden number is 10 and the Sunday letter is A. Now we check the calendar and find Easter Sunday.
The Golden Number is the numerical number to the left of the date in the calendar. Our golden number for the year 2023 is “10”, we need to check April.
The golden number “10” lands on April 5th, which is the first full moon after the Spring Equinox. Because our Sunday letter is “A”, we go to the following Sunday where A appears, which is April 9th. Easter Sunday falls on April 9th in the year of our Lord 2023.
Final Notes: The Spring or Vernal Equinox is the day in which the length of day and night are the same. Moving forward, past the Spring equinox, there is more daylight during the day, hence, Christ shines his light over the world.
There are 19 golden numbers and Easter must fall between Mar 21st and Apr 18th of every year.
Where did this calculation come from? – The Council of Nicea in 325 AD, The First Council of Nicaea, held in Nicea in Bithynia (in present-day Turkey), convoked by the Roman Emperor Constantine I in 325, was the first ecumenical conference of bishops of the Christian Church, and most significantly resulted in the first uniform Christian doctrine. With the creation of the Nicene Creed, a precedent was established for subsequent ‘general ( ecumenical) councils of Bishops’ (Synods) to create statements of belief and canons of doctrinal orthodoxy— the intent being to define unity of beliefs for the whole of Christendom — a momentous event in the history of the Church and subsequent history of Europe.
Links:
1928 Book of Common Prayer: http://justus.anglican.org/resources/bcp/1928/BCP_1928.htm
Spring Equinox information.
https://www.timeanddate.com/calendar/spring-equinox.html