- Why does Easter move every year?
- The First Council of Nicaea in 325 AD ruled that Easter is the first Sunday after the first ecclesiastical full moon on or after the spring equinox. The equinox is fixed to March 21 for this calculation, the full moon is calculated (not observed), and the Sunday after it is Easter. Because the full-moon cycle (29.5 days) doesn't line up with the week (7 days) or the year (365.25 days), the date moves anywhere from March 22 to April 25.
- Why is Orthodox Easter on a different day?
- Both traditions use the same rule (Sunday after the first full moon after the spring equinox) — they just compute it on different calendars. Western churches switched to the Gregorian calendar in 1582; Eastern Orthodox churches kept the Julian calendar's paschal computation. The Julian calendar drifts about 13 days behind Gregorian, and Orthodox tradition also requires Easter to fall after Jewish Passover, which pushes some years even later. Most years the two Easters are 1, 4, or 5 weeks apart; about once every 4 years they land on the same Sunday.
- When was the last time Western and Orthodox Easter were on the same day?
- 2025 — both fell on Sunday, April 20. Before that: 2017 (April 16), 2014 (April 20), 2011 (April 24), 2010 (April 4), 2007 (April 8), 2004 (April 11). The next alignment after 2025 is 2028 (April 16). On average it happens about every 3–5 years, depending on how the moon cycles align with each calendar.
- What's the earliest and latest Easter can be?
- Western (Gregorian) Easter ranges from March 22 (earliest) to April 25 (latest). March 22 last happened in 1818 and won't happen again until 2285. April 25 last happened in 1943 and next happens in 2038. The most common Easter dates are between April 1 and April 15.
- How are Ash Wednesday, Palm Sunday, and Good Friday computed?
- All three are fixed offsets from Easter Sunday. Ash Wednesday is 46 days before Easter — that's 40 fasting days plus 6 Sundays (which aren't counted as fast days). Palm Sunday is 7 days before Easter, the Sunday that opens Holy Week. Good Friday is 2 days before Easter, the Friday that commemorates the crucifixion. Pentecost is 49 days after Easter — 7 weeks counted inclusively.
- Which algorithm does this calculator use?
- Western Easter uses the Anonymous Gregorian algorithm, also called Butcher's algorithm or the Meeus / Jones / Butcher method — the same compact formula published in Jean Meeus's Astronomical Algorithms (1991) and adopted by most software libraries. Orthodox Easter uses Meeus's Julian-calendar paschal formula, with a Julian-to-Gregorian date conversion applied to give you the date your wall calendar would show.
- Does it work for years before 1583?
- No. The Gregorian calendar didn't exist before October 1582 — Catholic countries switched in 1582, Protestant countries gradually through the 17th and 18th centuries, and most Eastern Orthodox countries kept the Julian calendar into the 20th century. Asking 'what was the Gregorian Easter date in 1300?' is technically a proleptic calculation (extrapolating the rule backward), and historians prefer the Julian date that actually applied at the time. To keep the answer unambiguous, the calculator refuses years before 1583.
- What's the upper limit?
- 4099. The Meeus algorithm is documented as accurate through that year. Beyond it, the Gregorian century-leap-year corrections introduce a small drift that the formula doesn't perfectly model. If you need Easter for the year 5000 or 10000, you'll need a different (and more complex) astronomical method.
- Why is Pentecost 49 days after Easter, not 50?
- Pentecost is the 50th day of Easter when counted inclusively in the Christian liturgical calendar — Easter Sunday itself is day 1, so the 50th day lands 49 days later. The Greek word 'Pentecost' literally means 'fiftieth day.' Calendar arithmetic in modern terms says it's a 49-day offset; the ancient counting tradition says it's the 50th day. Both are saying the same thing.
- Is this the same as the date my Eastern Orthodox church will observe?
- Yes for any year — Greek Orthodox, Russian Orthodox, Serbian Orthodox, Romanian Orthodox, and Antiochian Orthodox churches all observe the same paschal date, derived from the Julian-calendar rule this calculator implements. Coptic Orthodox and Ethiopian Orthodox also align with this date most years. Western churches that have switched back to the Julian paschal rule (a small handful of traditional Catholic communities) would also use this date.
