How do they decide when Easter is?
Catholics and most Protestants use the new Gregorian calendar decreed by Pope Gregory XIII in 1582 to calculate the date of Easter.
The Greek Orthodox Church uses the old Julian calendar introduced in 46 B.C.E. by Julius Caesar.
For more information, see:
Catholic:
http://en.wikipedia.org/wiki/Gregorian_calendar
http://aa.usno.navy.mil/faq/docs/easter.html
Greek Orthodox:
http://en.wikipedia.org/wiki/Julian_calendar
http://www.smart.net/~mmontes/OrthEasttbl.html
Catholic:
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 2008 the vernal equinox will be on Thursday, March 20. The first full moon after that will be Friday, March 21. Thus, Easter will be on Sunday, March 23.
With love in Christ.