Why there are two Easter dates
Both traditions follow the same principle, set in the early centuries of the Church: Easter is the Sunday after the first full moon of spring. They apply it with different calendars and moon tables. Western churches adopted the Gregorian calendar and its revised tables in 1582. Most Orthodox churches still calculate Easter with the older Julian calendar and its tables, and the Julian calendar now runs 13 days behind the Gregorian one. As a result the two dates match in some years and differ by one, four or five weeks in others. This page does not present either date as more correct; each is shown as its own churches calculate it.
How the calculation works
Easter is worked out from a church (ecclesiastical) moon, a table moon that follows the real moon closely but not exactly, and from a spring equinox fixed on 21 March. The church full moon is the 14th day of a table month, and Easter is the first Sunday after the first church full moon on or after 21 March, so Western Easter always falls between 22 March and 25 April. The Western date here comes from the anonymous Gregorian algorithm, which gives the same results as the algorithm the US Naval Observatory publishes. The Orthodox date comes from Meeus's Julian algorithm and is then converted to today's civil calendar.
Days that move with Easter
Many days of the church year are counted from Easter Sunday. In the Western tradition, Lent begins on Ash Wednesday, 46 days before Easter. Palm Sunday is one week before Easter, followed by Holy Thursday and Good Friday. Ascension Day is a Thursday, 39 days after Easter Sunday; Pentecost is 49 days after, and Trinity Sunday one week later. In the Orthodox tradition, Great Lent begins on Clean Monday, 48 days before Easter (Pascha), and Holy Friday, Ascension and Pentecost follow the same spacing. Some dioceses move Ascension to the following Sunday, and local calendars add other days, so check your own church calendar.
Advent and the fixed feasts
The First Sunday of Advent is the fourth Sunday before Christmas, so it always falls between 27 November and 3 December. Christmas itself does not move with Easter. Churches that keep the Julian calendar for fixed feasts celebrate 25 December Julian, which is 7 January on the civil calendar from 1901 to 2099. Many Orthodox churches use the revised Julian calendar for fixed feasts and keep 25 December, while still calculating Easter by the Julian rule. The Ethiopian and Coptic churches have their own calendars, which our converter covers.
How the dates were checked, and the limits
Every Western date we tested, 37 sample years from 1583 to 4099 including Ash Wednesday, Ascension, Pentecost and Advent, matched the dates published by the US Naval Observatory. The Orthodox dates for 2001 to 2036 matched the table in the World Council of Churches' 1997 Aleppo consultation and current published lists. A few churches arrange things differently: for example, the Finnish Orthodox Church keeps Easter on the Western date. Years before 1583 are not offered, because the Gregorian calendar did not exist yet. This page is general help only and is not religious guidance; for services, fasting and customs, ask your own priest, pastor or parish.