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Computation of Date of Easter
Short Answer: Easter is the Sunday after the full moon1 on or after the vernal (or spring) equinox2. Easter occurs as early as March 22 and as late as April 25.
Extended Answer: The observant will notice two footnotes, because it just can't be that simple.
1. It's not exactly the familiar full moon, which most calendars mark when the astronomical full moon occurs (using Greenwich mean time, the universal time zone of Greenwich, England). Easter uses the Paschal Full Moon, which is computed rather than observing the moon. Originally, tables were used. For 2008, the PFM is March 22, and the AFM is March 21 18:41 GMT (13:41 EST). The PFM and AFM occasionally differ by one or two days.
2. It's not exactly the familiar vernal equinox, which most calendars mark when the Sun crosses directly over the equator (again, using Greenwich mean time), varying from March 19–21. Easter uses the fixed date of March 21. The astronomical vernal equinox for 2008 is March 20 05:58 GMT (00:58 EST).
A formula to compute the date of Easter is provided below. <formula>
The following feasts and observances are related to Easter:
| Holy Day | Relationship to Easter | Date for 2008 |
| Septuagesima | 63 days before | January 20, 2008 |
| Quinquagesima | 49 days before | February 3, 2008 |
| Ash Wednesday (Beginning of Lent) | 46 days before (often stated as 40 days, but that excludes Sundays) | February 6, 2008 |
| Palm Sunday | 7 days before | March 16, 2008 |
| Holy Thursday (Triduum begins with Mass of the Lord's Supper) | 3 days before | March 20, 2008 |
| Good Friday | 2 days before | March 21, 2008 |
| Ascension Thursday (moved to next Sunday in some locations) | 39 days after | May 1, 2008 |
| Pentecost | 49 days after | May 11, 2008 |
| Monday after Pentecost (Ordinary Time resumes) | 50 days after | May 12, 2008 |
| Trinity Sunday | 56 days after | May 18, 2008 |
| Corpus Christi (moved to previous Thursday in some locations) | 63 days after | May 25, 2008 |
| Sacred Heart | 68 days after | May 30, 2008 |
Algorithm by Jean Baptiste Joseph Delambre (1749-1822) to compute date of Easter
works for the Gregorian calendar (the one we use every day) since 1583
| Computation | Example | |
| / means integer division without a remainder | 13 / 4 = 3 8 / 2 = 4 |
|
| % means modulo division or remainder | 13 % 4 = 1 8 % 2 = 0 |
|
| y is 4-digit year | ||
|
a = y % 19 |
13 | |
|
b = y / 100 |
20 | |
| c = y % 100 | 8 | |
|
d = b / 4 |
5 | |
| e = b % 4 | 0 | |
|
f = (b + 8) / 25 |
1 | |
| g = (b – f + 1) / 3 | 6 | |
|
h = (19 × a + b – d – g + 15) % 30 |
1 | |
| i = c / 4 | 2 | |
|
k = c % 4 |
0 | |
| l = (32 + 2 × e + 2 × i – h × k) % 7 | 0 | |
|
m = (a + 11 × h + 22 × l) / 451 |
0 | |
| r = 22 + h + l – 7 × m | 23 | |
| If r < 32, then r is day of March otherwise (r – 31) is day of April |
March 23, 2008 |