Pascha vs. Easter Date Calculator

Both traditions follow the same rule inherited from the First Council of Nicaea in 325: Pascha falls on the first Sunday after the first full moon on or after the vernal equinox. They diverge on two points of reckoning, not on the rule itself.

The calendar. The Western computus reckons from the Gregorian calendar, adopted in 1582. Most Orthodox churches reckon the Paschalion from the older Julian calendar, which now runs thirteen days behind. So the Julian “March 21” equinox anchor actually falls on April 3 by civil reckoning.

The moon. The Gregorian reform also corrected the tables used to predict the paschal full moon, which had drifted several days off the true astronomical moon. The Julian Paschalion retains the older nineteen-year lunar cycle.

Because both always land on a Sunday, the gap between them is always a whole number of weeks — in practice zero, one, four, or five. There is a further constraint on the Orthodox side: Pascha must follow Jewish Passover, which is why the gap sometimes widens to a full month.