Please tell me when exactly Easter falls and how to calculate it?

Easter Sunday is a moveable feast whose date is determined by a specific lunisolar calculation tied to the March equinox and the phases of the moon. According to the standard Gregorian calendar computation used by most Western Christian churches, Easter falls on the first Sunday after the first ecclesiastical full moon that occurs on or after March 21. This ecclesiastical full moon is a formal calculation, not always perfectly aligned with the astronomical full moon, and March 21 is fixed as the ecclesiastical approximation of the vernal equinox. Consequently, Easter can occur as early as March 22 and as late as April 25. For example, in 2024, the first ecclesiastical full moon after March 21 was calculated to be on March 25, making the following Sunday, March 31, Easter Sunday.

The calculation's mechanism is rooted in the desire to link Easter to the Jewish Passover, which is based on the lunar Hebrew calendar, while ensuring it always falls on a Sunday, the day of the Resurrection. The process involves determining the "Paschal Full Moon" for each year using a 19-year Metonic cycle, which approximates the time it takes for the lunar phases to recur on the same calendar dates. A set of tables and formulas, such as the Gauss algorithm, can compute the date precisely. In practice, one finds the "Golden Number" for the year (its position in the 19-year cycle), uses it to find the date of the Paschal Full Moon from pre-calculated tables, and then locates the following Sunday. This complex system was formalized at the First Council of Nicaea in 325 AD and later refined with the Gregorian calendar reform in 1582 to correct for calendar drift.

It is crucial to note that many Eastern Orthodox churches calculate Easter using the older Julian calendar for the fixed date of the equinox (April 3 on the Gregorian calendar) and a different base for the lunar calculations. This often, but not always, results in a later celebration, and in some years the dates coincide, as they will in 2025 when both traditions celebrate on April 20. The calculation's complexity means that for most practical purposes, individuals and institutions rely on published tables or algorithmic calculators rather than manual computation. The specific date for any given year is a fixed point once calculated, not subject to annual astronomical observation.

The implications of this dating system are significant, as it governs the entire liturgical calendar for Christianity, setting the timing of movable observances like Ash Wednesday, Pentecost, and Trinity Sunday. It also creates a variable relationship with civil calendars, affecting planning for secular activities like school holidays and commercial seasons. The enduring use of this computus, despite proposals for a fixed date, underscores the deep historical and theological weight carried by the link to Passover and the lunar cycle, embedding a layer of ancient calendrical science into the rhythm of modern religious life.