Dionysius Exiguus invented the *Anno Domini* era to calculate the Easter date.
**Computus** (Latin for "computation") is the method for calculating the date of Easter. In the early 4th century there was confusion about when Christian Easter or the Resurrection should be celebrated.

History
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### Background

At the Arles Council, in 314, was obliged to all the Christianity to celebrate Easter the same day, and that this date was going to be assigned by the Pope, that was going to send epistles to all the churches in the world with the necessary instructions. However, not all the congregations followed those precepts.

### Nicaea Council

In the First Council of Nicaea, in 325, this subject was solved.

Was established that the Resurrection Easter had to be celebrated fulfilling some norms:

* Easter had to be celebrated on Sunday.
* It can not coincide with the Jewish festival of Passover, celebrated independently which day of the week it was.
* Christians cannot celebrate Easter twice the same year. This is explained because the new year started in the spring equinox, so the celebration of Easter before the real equinox was prohibited.

However, there still were discussions between the Roman Church and the Alexandrian Church, while the Nicaea Council said that the Alexandria Church was right, they had to calculate the date (in Alexandria), then communicate it to Rome. The Roman Church had to spread the *computus* to the rest of the Christianity.

Calculation
-----------

|
|  |
Dates for Easter   
 1982–2022  
In Gregorian dates| Year | Western | Eastern |
| --- | --- | --- |
| 1982 | April 11 | April 18 |
| 1983 | April 3 | May 8 |
| 1984 | April 22 |
| 1985 | April 7 | April 14 |
| 1986 | March 30 | May 4 |
| 1987 | April 19 |
| 1988 | April 3 | April 10 |
| 1989 | March 26 | April 30 |
| 1990 | April 15 |
| 1991 | March 31 | April 7 |
| 1992 | April 19 | April 26 |
| 1993 | April 11 | April 18 |
| 1994 | April 3 | May 1 |
| 1995 | April 16 | April 23 |
| 1996 | April 7 | April 14 |
| 1997 | March 30 | April 27 |
| 1998 | April 12 | April 19 |
| 1999 | April 4 | April 11 |
| 2000 | April 23 | April 30 |
| 2001 | April 15 |
| 2002 | March 31 | May 5 |
| 2003 | April 20 | April 27 |
| 2004 | April 11 |
| 2005 | March 27 | May 1 |
| 2006 | April 16 | April 23 |
| 2007 | April 8 |
| 2008 | March 23 | April 27 |
| 2009 | April 12 | April 19 |
| 2010 | April 4 |
| 2011 | April 24 |
| 2012 | April 8 | April 15 |
| 2013 | March 31 | May 5 |
| 2014 | April 20 |
| 2015 | April 5 | April 12 |
| 2016 | March 27 | May 1 |
| 2017 | April 16 |
| 2018 | April 1 | April 8 |
| 2019 | April 21 | April 28 |
| 2020 | April 12 | April 19 |
| 2021 | April 4 | May 2 |
| 2022 | April 17 | April 24 |

Let's define 5 variables, *a*, *b*, *c*, *d*, and *e*. In addition to two constants, *M* and *N*, so the years between 1900 and 2100 take the values 24 and 5, respectively. We'll call *A* the year that we want to calculate the Easter date.

*a* is the remainder of the division {\displaystyle {\frac {A}{19}}}, or technically according to the modular arithmetic we should say {\displaystyle A\ mod\ 19},
*b* is the remainder of the division {\displaystyle {\frac {A}{4}}},
*c* is the remainder of the division {\displaystyle {\frac {A}{7}}},
*d* is the remainder of the division {\displaystyle {\frac {19a+M}{30}}},
*e* is the remainder of the division {\displaystyle {\frac {2b+4c+6d+N}{7}}}.
If *d* + *e* < 10, the Easter date will be in March (*d* + *e* + 22). If opposed (*d* + *e* > 9), will be in April (*d* + *e* − 9).

There are 2 exceptions:

* If the obtained date is April 26, the Easter date will be April 19, **not** April 26.
* If the obtained date is April 25, with *d* = 28, *e* = 6 and *a* > 10, then the Easter date will be April 18.

The values for M and N for years before 1900 or after 2100 can be obtained 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 |

Further reading
---------------

* Mosshammer, Alden A. *The Easter Computus and the Origins of the Christian Era*. Oxford: Oxford University Press, 2008. ISBN 0-19-954312-7.