- Determine the date of first full Moon on or after the first day of Spring.
- Easter is the Sunday after that.
Except that it doesn't. A lunar eclipse can occur only when the Moon is full, and there was a partial lunar eclipse on Easter Sunday, April 12, 1903.
There are two factors in the rule for Easter that contribute to this situation.
- The first day of Spring is always assumed to be March 21. This is not exactly true. The vernal equinox (determined astronomically) can occur on March 19, 20 or 21.
- The full Moon used is not the astronomical full Moon, but an ecclesiastical full Moon, i.e., one that is used for ease of determination.
The date of Easter is actually determined by the Roman Catholic Church using tables that were set up when the Gregorian Calendar was established in 1582.
For further information: http://aa.usno.navy.mil/faq/docs/easter.php