About the Easter Calculator
Easter is a "movable feast" — unlike Christmas, it does not fall on a fixed calendar date. It is defined as the first Sunday after the Paschal Full Moon, the first ecclesiastical full moon occurring on or after March 21. Because tracking the actual moon is impractical for a fixed liturgical calendar, churches instead use a deterministic arithmetic procedure called the computus to compute the date years or centuries in advance. This calculator implements the standard computus algorithms for both the Western (Gregorian) and Orthodox (Julian) traditions.
The Gregorian computus (Western Easter)
For years 1583 onward, Western churches use the Anonymous Gregorian algorithm (also known as the Meeus/Jones/Butcher algorithm), a compact set of modular-arithmetic steps that reproduces the ecclesiastical full-moon tables exactly. Given a year Y, the algorithm computes intermediate values from Y mod 19 (the year's position in the 19-year Metonic lunar cycle), Y's century, and several correction terms, then resolves them into a month and day in March or April. The result always falls between March 22 and April 25.
The Julian computus (Orthodox Easter)
Most Orthodox churches instead compute Easter (Pascha) using the Julian calendar and an older computus. This calculator uses the standard Julian algorithm to get the Julian-calendar month and day, then converts that date to the Gregorian calendar we use in everyday life by adding the current Julian-to-Gregorian offset — 13 days as of the 1900s through 2099, widening by roughly one day per century. Because the Julian and Gregorian calendars have drifted apart and the two traditions approximate the lunar cycle differently, Orthodox Easter coincides with Western Easter in some years and falls one to five weeks later in others.
Related movable feasts
Once Easter Sunday is fixed, several other observances follow by a constant day offset in both traditions:
- Palm Sunday — 7 days before Easter, marking the start of Holy Week.
- Good Friday — 2 days before Easter, commemorating the crucifixion.
- Ash Wednesday (Western only) — 46 days before Easter, the start of the 40 days of Lent (excluding Sundays).
- Pentecost Sunday — 49 days after Easter, the 50th day of the Easter season counting Easter itself as day one.
Practical notes
The Gregorian calendar was not adopted until 1582, so this calculator restricts input to years 1583 and later, where the computus formulas are unambiguous. Dates are civil calendar dates (day of the week is always Sunday for Easter, Palm Sunday, and Pentecost by construction); no time zone or astronomical observation is involved, since the ecclesiastical full moon is a calculated approximation, not the actual full moon.