Easter Sunday is determined using the Computus algorithm, a calculation established by the Council of Nicaea in 325 AD. The rule states that Easter falls on the first Sunday after the first full moon on or after the spring equinox (March 21).

This means Easter can occur anywhere between March 22 and April 25 — a 35-day window. The algorithm uses a complex set of calculations involving the Metonic cycle (19-year lunar cycle), epacts, and golden numbers to determine the Paschal full moon date without direct astronomical observation.

The difference stems from two factors:

• Calendar systems: Western churches use the Gregorian calendar (introduced by Pope Gregory XIII in 1582), while many Orthodox churches still use the Julian calendar for liturgical calculations.
• Equinox reference: The Julian calendar's March 21 currently falls 13 days later than the Gregorian March 21 (this gap increases over centuries).

As a result, Orthodox Easter is usually 1 to 5 weeks later than Western Easter, though they occasionally coincide (most recently in 2017, and will again in 2025, 2028, 2031, and 2034).

The Computus (Latin for "computation") is the mathematical algorithm used to calculate the date of Easter. Our tool uses the Butcher's Algorithm (also known as the Meeus/Jones/Butcher method) for Gregorian Easter, which is the most widely accepted implementation.

The algorithm is 100% accurate for all years from 1583 onward under the Gregorian calendar. It correctly handles all edge cases, including the rare situation where the calculated date would fall on April 26, which the algorithm automatically adjusts to April 19 (as occurred in 1981).

Under the Gregorian calendar, Easter Sunday can fall on:

• Earliest: March 22 — This is extremely rare. The last time was in 1818, and the next occurrence will be in 2285.
• Latest: April 25 — Also rare. The last occurrence was in 1943, and the next will be in 2038.

The most common Easter date is April 19, which occurs about 3.9% of the time over the complete 5,700,000-year Gregorian Easter cycle.

Many important Christian observances are calculated relative to Easter Sunday:

• Ash Wednesday — 46 days before Easter (marks the start of Lent)
• Palm Sunday — 7 days before Easter
• Maundy Thursday — 3 days before Easter
• Good Friday — 2 days before Easter
• Ascension Day — 39 days after Easter (always a Thursday)
• Pentecost — 49 days after Easter (always a Sunday)
• Trinity Sunday — 56 days after Easter
• Corpus Christi — 60 days after Easter (always a Thursday)

All these dates are automatically calculated and displayed when you search for any year using our Easter Date Finder.

Easter's date varies because it depends on the lunar calendar. Since the full moon can occur on different dates relative to the spring equinox (March 21), the following Sunday shifts accordingly. The full moon used in the calculation is the Paschal full moon — an ecclesiastical approximation rather than the astronomical full moon.

If the Paschal full moon falls on a Sunday, Easter is pushed to the following Sunday to maintain the "first Sunday after" rule. This interplay between lunar cycles, the equinox, and the weekly cycle creates the 35-day possible range.

Our tool is optimized for the Gregorian calendar era (1583 onwards). For years before 1583, the Gregorian calendar did not exist — Europe used the Julian calendar exclusively. While the algorithm can mathematically produce a result for earlier years, the date would not correspond to any historically observed Easter, as the Gregorian reform had not yet occurred.

For pre-1583 dates, historians typically use the Julian Computus algorithm exclusively. If you need Easter dates for medieval or ancient history research, specialized historical chronologies are recommended.

Western and Orthodox Easter fall on the same day approximately 25-30% of the time. Recent and upcoming shared Easter dates include:

• 2017 — April 16
• 2025 — April 20
• 2028 — April 16
• 2031 — April 13
• 2034 — April 9

These coincidences occur when the Julian calendar's Paschal full moon aligns closely enough with the Gregorian calculation, despite the calendar offset.