The first council of Nicea established Easter as the first Sunday after the first full moon that falls on or after the Vernal Equinox. But all Orthodox Christians use the old calendar to calculate the date of the Equinox. (See my post on the Old Calendar.) This means that the Old Calendar and New Calendar Orthodox are united for the Paschal season (from the start of Great Lent through Pentecost), even though they are 13 days off for the liturgical calendar and the rest of the great feasts.
During 2010 and 2011 Easter and Pascha fell on the same day. This year and for the next few years the dates are different. Here are the dates:
|2012||April 8||April 15|
|2013||March 31||May 5|
|2014||April 20||April 20|
|2015||April 5||April 12|