Easter is calculated to follow Passover which is based on the lunar calendar.
In 2009, Easter will be on Sunday, April 12.
Catholics and most Protestants use the new Gregorian calendar decreed by Pope Gregory XIII in 1582 to calculate the date of Easter.
For more information, see:
http://en.wikipedia.org/wiki/Gregorian_calendar
http://aa.usno.navy.mil/faq/docs/easter.php
The time of Easter is based on the cycles of the moon, the equinox, and seasons and things in nature.
In simple (?) terms Easter occurs on the first Sunday following the first full moon that occurs on or after the day of the vernal equinox (the first day of spring).
In 2009 the vernal equinox will be on Friday, March 20. The first full moon after that will be Thursday, April 9. Thus, Easter will be on Sunday, April 12.
The earliest Easter can be is March 22 as it was last in 1818 and will be next in 2285.
The latest Easter can be is April 25 as it was last in 1943 and will be next in 2038.
With love in Christ.