How Is the Date of Easter Determined?

The rule, set in 325 CE

The method for calculating Easter's date traces back to the First Council of Nicaea in 325 CE, convened by the Roman Emperor Constantine. The Council decided that Easter should fall on the same Sunday throughout the entire Christian world, and constructed tables to compute that date far in advance rather than relying on direct observation of the Moon each year. Those tables were refined over the following centuries, most notably by the 6th-century Abbot Dionysius Exiguus, and again in 1582 when Pope Gregory XIII's calendar reform (the Gregorian calendar still used today) produced new Easter tables.

The exact ecclesiastical rule

Easter Sunday is the first Sunday following the first ecclesiastical full moon that occurs on or after March 21. Every word here is doing specific work:

• March 21 is a fixed date, not the real astronomical equinox (which actually drifts slightly, typically falling on March 19–21). The Church deliberately fixed it so the calculation would never depend on year-to-year astronomical observation.
• Ecclesiastical full moon means a full moon date taken from a set of church tables, not the date of the real, observable full moon. The tables track the Moon's cycle closely but not exactly, so the ecclesiastical and astronomical full moon occasionally fall a day apart.
• First Sunday after that full moon: never the same day as the full moon itself, always the Sunday that follows it.

The Golden Number and the 19-year cycle

The traditional shortcut for finding a year's ecclesiastical full moon is the Golden Number: a year's position in the 19-year Metonic cycle, the cycle after which the Moon's phases repeat on approximately the same calendar dates. To find it, add 1 to the year and divide by 19: the remainder is the Golden Number (if there is no remainder, it is 19). For example, the Golden Number for 2026 is 13. Historically, a Golden Number was cross-referenced against a printed table to find that year's Paschal Full Moon date; the modern algorithmic version (the one this site's calculator uses, verified against published Easter dates through 2029) folds the same 19-year cycle directly into the formula rather than a lookup table.

What happens in an edge-case year

Because the ecclesiastical equinox is fixed at March 21 rather than tracking the real astronomical equinox, the two occasionally diverge in a way that changes the outcome. In 2019, for example, both the astronomical equinox and the astronomical full moon fell on March 20, but since the Church's equinox is fixed at March 21, that full moon did not count as the Paschal Full Moon under the ecclesiastical rule, and Easter was instead observed on April 21, following the next full moon. This is a genuine, if rare, way the ecclesiastical calculation can differ from what a purely astronomical calculation would produce.

Western vs. Eastern Orthodox calculation

Western churches (Catholic and Protestant) apply this rule using the Gregorian calendar's leap-year system, adopted in 1582. Many Eastern Orthodox churches still calculate the same underlying rule using the older Julian calendar, which has a different (and, by now, less accurate) leap-year pattern. That difference in calendars, not a different theological rule, is what usually puts Orthodox Easter one to five weeks after the Western date. A 1923 congress of Eastern Churches adopted a modified calendar for other purposes, but a variety of practices for Easter's date remain among Orthodox churches to this day. See our Easter date calculator for both dates side by side in any year, or the full year-by-year table.

Working example: how 2026's date was found

Applying the rule above to 2026: 2026's position in the 19-year lunar cycle (Golden Number 13) places the ecclesiastical Paschal Full Moon on a specific date after March 21; the first Sunday following that full moon is April 5, 2026. Every year's date is found the same way; only the position in the 19-year cycle, and the resulting full-moon date, changes.