I blogged about this a year ago, but thought it would be good to revisit the relationship between Easter and astronomy.
The date of Easter is commonly explained as the Sunday after the Full Moon on or after the vernal equinox in the northern hemisphere. The vernal equinox can occur on either March 19th, 20th or 21st. The date of the subsequent Full Moon, however, can fluctuate.
Our calendar is a solar calendar, being based on the position of the Sun against the background stars. Therefore, the lunar phase cycle, being 29 1/2 days long, will appear to wander across the calendar, and the date of the first Full Moon on or after the equinox could be the same day as the equinox or up to 4 weeks later.
The date of Easter ranges from March 22nd to April 25th, but the extreme dates are fairly rare. In 2008, Easter fell on March 23rd. In 2011, Easter will fall on April 24th. The extreme dates are pretty far away. Easter will not occur on March 22nd until 2285, or on April 25th until 2038.
If this calculation seems confusing enough, Eastern Christianity uses slightly different definitions for the equinox and the Full Moon. These subtle differences can result in very different dates for Easter. In 2009, countries and cultures following the eastern orthodox traditions celebrated Easter on Sunday, April 19th–one week later than western tradition. In 2010, the two dates coincide on April 4th. In 2013 there will be 5 weeks between the two Easter dates: March 31st for western cultures and May 5th for eastern cultures.