Catholics and Orthodox Christians do not celebrate two different Easters. They are calculating the same feast in two different ways. The basic rule has been shared since the Council of Nicaea: Easter must be celebrated after the spring equinox, after the Paschal full moon, and on a Sunday. The split appears later, in the calendars and calculation tables used to turn that rule into a date.
Explanation
Western churches calculate Easter by the Gregorian calendar and the Western computus. Most Orthodox churches still use the Julian Paschalion for Pascha, even if some of them already use the Revised Julian calendar for fixed feasts. Because of that, the “ecclesiastical” spring equinox and the “ecclesiastical” full moon often fall on different days in the Eastern and Western traditions.
This creates an important paradox: moving fixed-feast observance to the Revised Julian calendar does not automatically mean changing the Paschalion as well. For Easter, many Orthodox churches deliberately keep the traditional method of calculation so they do not break common Pascha observance with one another.
Details
It is important not to confuse church calculation with real-time astronomy. Both Catholics and Orthodox Christians usually determine Easter by calendrical rules and paschal tables, not by directly observing the sky each year. The difference is that Western tables are tied to the Gregorian calendar, while the Eastern calculation traditionally follows the Julian one. In the twenty-first century, March 21 on the Julian calendar corresponds to April 3 on the civil Gregorian calendar, so the Orthodox calculation is often pushed later from the start.
It also helps to explain the phrase “Paschal full moon.” This is not necessarily the same full moon you happen to see in the sky that night. In church calculation it is a computed calendar date. In the Eastern tradition, the paschal tables are linked to the 19-year Metonic cycle rather than to direct modern observatory data. That is why the astronomical full moon and the ecclesiastical Paschal full moon do not always match exactly.
That is why the dates do not differ every year. Sometimes they coincide, when both systems lead to the same Sunday after the relevant Paschal full moon. In other years, Western and Eastern Easter can be separated by a week or even more.
What about Passover?
You will often hear that Orthodox Pascha “must always come after Jewish Passover.” That explanation is popular, but it is too simple. Historically, the Church developed its own way of computing Pascha and does not directly anchor the date to the modern Jewish calendar. At the same time, the Eastern tradition does preserve a principle of not celebrating Pascha earlier than, or at the same time as, Passover, which is one reason the Orthodox date is often later in practice. So Passover matters to the tradition, but the main source of the date gap is still the use of different calendars and different paschal tables.
In short
- The rule is shared: after the spring equinox, after the Paschal full moon, and on a Sunday.
- The “Paschal full moon” is a computed date: it does not always match the full moon you see in the sky, because the church uses paschal tables.
- The Revised Julian calendar does not automatically change Easter: fixed feasts may move, while Pascha is still calculated separately.
