It’s March of 2025 and we all know Easter is near. Most of us though can only say it’s “near” or in the “spring”. We can say that the start of spring is on March 21st, Independence Day in on July 4th and Christmas Day is on December 25th. While Thanksgiving Day isn’t always a specific day, it is always on the fourth Thursday of November. Easter though requires much more understanding to know when we will celebrate the resurrection of Jesus.
Easter Sunday is determined based on the “ecclesiastical approximation of the lunar calendar.” Or better, the first Sunday after the first full moon following the vernal equinox which occurs around March 21 (when we declare the start of Spring).
Let’s make this even more mysterious:
- The vernal equinox is set as March 21, even though the actual equinox might occur a day or two earlier or later.
- The first full moon after March 21 is identified.
- Easter Sunday is the Sunday immediately following that full moon.
This method of calculating Easter Sunday came from the Council of Nicaea in 325 AD. The aim was to standardize the date for all Christians. Since the lunar cycle varies doesn’t sync perfectly with our 365.25-day year, Easter can fall anywhere between March 22 and April 25. Over a month of variance!
Last year, Easter Sunday was on March 31st. This year it’s on April 17th. And next year it’s April 5th. Given these variations, it’s always best to plan your Easter services and activities early!
There are though some mathematical algorithms that attempt to help anyone calculate Easter Sunday. Take Jean Meeus’s Astronomical Algorithms (1991). This is a definitive modern source for the Gregorian Computus, providing a clear mathematical formula. It’s not “official” in a religious sense but is authoritative for its precision and alignment with Church practice.
For example, Meeus’s algorithm (simplified):
- a=year mod 19
- b=⌊year/100⌋
- c=year mod 100
- d=⌊b/4⌋
- e=b mod 4
- f=⌊(b+8)/25⌋
- g=⌊(b−f+1)/3⌋
- h=(19a+b−d−g+15) mod 30
- i=⌊c/4⌋
- k=cmod 4
- l=(32+2e+2i−h−k)mod 7
- m=⌊(a+11h+22l)/451⌋
- Month = ⌊(h+l−7m+114)/31⌋ (3 = March, 4 = April)
- Day = [(h+l−7m+114)mod 31]+1
Let’s just leave that there and celebrate the resurrection of Jesus!
Happy Easter!
