This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
The Greek Orthodox Church does not always celebrate Easter on the same
day 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
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 the basis for calculating Easter,
rather than the "official" full moon described in the previous
sections. And they chose the meridian of Jerusalem to serve as
definition of when a Sunday starts. However, except for some sporadic
use the 1920s, this system was never adopted in practice.