How Is the Date of Easter Calculated?

Apr 22, 2011 by

Easter is a moveable feast, which means that it does not occur on the same date every year.

The Council of Nicaea (A.D. 325) set the date of Easter as the Sunday following the paschal full moon, which is the full moon that falls on or after the vernal (spring) equinox.

The reason being is that the paschal full moon is the date of Passover in the Jewish calendar, and the Last Supper (Holy Thursday) occurred on the Passover. Christ was crucified on the Friday and was resurrected on the Sunday. Therefore, Easter is the Sunday after Passover.

The Church does not use the exact date of the paschal full moon but an approximation, because the paschal full moon can fall on different days in different time zones, which would mean that the date of Easter would be different depending on which time zone you live in.

Easter Dates

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Date  24 April 8 April 31 March 20 April 5 April 27 March 16 April 1 April 21 April 12 April
2021 2022 2023 2024 2025 2026 2027 2028 2029 2030
Date  4 April 17 April 9 April 31 March 20 April 5 April 28 March 16 April 1 April 21 April



Algorithm to Calculate Easter

The detailed algorithm which is used by the Western Church (Roman Catholic and Protestant) is as follows:-

' Given the Year as a four digit number in the range 1700 - 2299
' Calculate Day and Month of Easter Sunday
' Note 1: the \ operator performs integer division without remainder.
' Note 2: the date returned is the Gregorian Calendar date
'         (the one we use now), even for dates in the 18th Century.

a = Year mod 19;
b = Year \ 100;
c = Year mod 100;
d = b \ 4;
e = b mod 4;
f = c \ 4;
g = c mod 4;

h = (b + 8)\25;
i = (b - h + 1)\3;
j = (19*a + b - d - i + 15) mod 30;
k = (32 + 2*e + 2*f - j - g) mod 7;
m = (a + 11*j + 22*k) \ 451;
n = j + k - 7*m + 114;

Month = n\31;
Day = (n mod 31) + 1;