Pascha vs. Easter Date Calculator

All three columns 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. Roman Catholics, and most Byzantine Catholics in the U.S., reckon from the Gregorian calendar, adopted in 1582. Most Orthodox churches — and some Eastern Catholic jurisdictions — reckon the Paschalion from the older Julian calendar, which currently runs thirteen days behind (true from 1900 through 2099; it was ten days in 1583 and the gap keeps widening). So the Julian “March 21” equinox anchor currently 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.

Byzantine Catholics fall on both sides. The movable cycle itself — pre-Lenten Sundays, Great Lent, Bright Week — is the same either way; only the anchor date changes (with one exception: whether Zacchaeus Sunday gets a Sunday of its own depends on where Pascha falls relative to Theophany). Most Byzantine Catholic parishes in the United States use the Gregorian date; parishes on the Julian calendar, including much of Ukraine, use the other.

Because all three always land on a Sunday, the gap between any two is always a whole number of weeks. From 1583 through 2099 it is always zero, one, four, or five. As the two calendars drift further apart the pattern shifts: four-week gaps end after 2092, and in the far future six-week and two-week gaps appear (first in 2437 and 2725).

Why the gap sometimes widens to a month. The Julian reckoning counts full moons only from its own March 21 (April 3 on the civil calendar). When the paschal full moon falls just before that date, as it can in years when Western Easter is early, Orthodox Pascha has to wait for the next full moon, a lunar month later. The Orthodox canons also say Pascha should follow Jewish Passover, but the Julian Paschalion is a fixed arithmetic table and does not consult the Jewish calendar.