Greek & Cretan Easter
The Greek Orthodox Church does not always celebrate Easter on the same date as the Catholic and Protestant countries. The reason is that the Orthodox Church uses the Julian calendar when calculating Easter. This is case even in the churches that otherwise use the Gregorian calendar. When the Greek Orthodox Church in 1923 decided to change to the Gregorian calendar (or rather: a Revised Julian Calendar), they chose to use the astronomical full moon as seen along the meridian of Jerusalem as the basis for calculating Easter, rather than to use the "official" full moon.
2000: April 30 2001: April 15 2002: May 5
2003: April 27 2004: April 11 2005: May 1
2006: April 23 2007: April 8 2008: April 27
Information for this answer was copied - with grateful thanks - from Greek & Cretan Easter customs. Click on this link for a full - and fascinating - background to this most important festival.