## 2008-03-20

### Easter: The holiday I just don't quite get

Well, okay, let me clarify the title... I know what Easter is about

The thing I never understood was why it was held when it was held: and by "when it was held" I mean "why its apparently some random date between March 15th and April 25th".

Luckily for us, in this modern day there's Wikipedia:

The canonical rule is that Easter day is the first Sunday after the 14th day of the lunar month (the nominal full moon) that falls on or after 21 March (nominally the day of the vernal equinox). For determining the feast, Christian churches settled on a method to define a reckoned "ecclesiastical" full moon, rather than observations of the true Moon as the Jews did at the time. Eastern Orthodox Christians calculate the fixed date of 21 March according to the Julian Calendar rather than the modern Gregorian Calendar, and observe the additional rule that Easter may not precede or coincide with the first day of the Jewish Passover.

Finally, for those mathematically inclined:
This algorithm for calculating the date of Easter Sunday was first presented by the mathematician Carl Friedrich Gauss:

The number of the year is denoted by Y; mod denotes the remainder of integer division (e.g., 13 mod 5 ≡ 3; see modular arithmetic). Calculate first a, b, and c:

a = Y mod 19
b = Y mod 4
c = Y mod 7

Then calculate

d = (19a + M) mod 30
e = (2b + 4c + 6d + N) mod 7

For the Julian calendar (used in Eastern churches), M = 15 and N = 6, and for the Gregorian calendar (used in Western churches), M and N are from the following table:

Years M N
1583-1699 22 2
1700-1799 23 3
1800-1899 23 4
1900-2099 24 5
2100-2199 24 6
2200-2299 25 0

If d + e < d =" 28," e =" 6,"> 10, Easter is on 18 April.