Sigh. It's time for that annual round of ABSOLUTELY WRONG "Why Orthodox Pascha [Easter] doesn't line up with Western Easter every year" articles.
It's very simple. The basic principle is: Pascha happens the first Sunday following the first full moon following the vernal (spring) equinox, which is assumed as the invariant date of March 21. This was established by the First Ecumenical Council, held in Nicaea in AD 325.
The differences arise because:
The Orthodox use March 21 on the Julian (Old) Calendar, which differs by some 13 days from the Gregorian (New) Calendar. If a full...