Did you notice that Orthodox Pascha doesn’t usually fall on the same day as everyone else’s? There’s a good reason for this. The Orthodox Christian Church calculates a bit differently than other Christian denominations. This isn’t done to confuse everyone. There are a few really good reasons why it may fall on a different date.
The Orthodox Churches around the world calculate the date of Pascha using the Julian Calendar. The reason is that some of the Orthodox Churches follows the Julian Calendar (p.e.Patriarchate of Jerusalem, Church of Russia) and to prevent the schisms all the Orthodox Churches calculate the date of Pascha with the Julian Calendar (and use also a different cycle for the calculation of full moon) so that to celebrate on the same day.
The 4 prerequisites for the calculation of the date of Pascha:
1. The Vernal Equinox (always on March 21st with the Julian Calendar/April 3rd with the Gregorian Calendar) (A’ Ecumenical Council)
2. First Full Moon after the Vernal Equinox (different cycle for calculating the full moon) (A’ Ecumenical Council)
3. Not before or with the Jewish Passover (7 Canon of the Apostolic Canons)
4 Always Sunday (A’ Ecumenical Council)
This year (2015) the Orthodox Pascha is on Sunday, April 12.