The vast majority of the Christian world celebrates Easter next weekend, but not all of Europe celebrates it on the same date. While for Roman Catholics and Protestants, Easter Sunday is on 5 April, for the Orthodox of various denominations, the celebration takes place a week later.
The Catholic Church uses the Gregorian calendar, and the Orthodox Church follows the Julian calendar. The difference between these calendars today is 13 days, which affects the calculation of dates.
Both churches follow the rule from the First Ecumenical Council of Nicaea – Easter should be celebrated on the first Sunday after the first full moon after the vernal equinox, but Catholics calculate the equinox according to the Gregorian calendar, and the Orthodox according to the Julian one.
The Orthodox Church adds another rule, that Easter must be after the Jewish Passover (Pesach), which often moves the date later than Catholic Easter.
The Orthodox Church conducts its liturgical life according to the Julian calendar, popularly called the ‘old calendar’ or ‘old style’. That calendar is now 13 days behind the ‘new’ Gregorian calendar, which is in most widespread use in the world.
