The date of Easter is a "movable feast," and its calculation is one of the most complex and historically significant problems in the Western calendar. Known as the *Computus*, the calculation determines not only Easter but the entire liturgical season surrounding it, including Lent and Pentecost.

The Three Rules: Why Easter Changes Every Year

Set by the First Council of Nicaea in 325 CE, the rule for Easter is: it falls on the first Sunday after the first full moon that occurs on or after the vernal equinox.

• The Vernal Equinox: The start of spring, fixed for ecclesiastical purposes as March 21.
• The Paschal Full Moon: This is not the astronomical full moon but an ecclesiastical approximation based on the 19-year Metonic cycle.
• The Sunday Rule: Easter must fall on a Sunday.

Julian vs. Gregorian Easter: The 13-Day Divide

A major point of division is that the Western Christian Church uses the Gregorian calendar for this calculation, while most Eastern Orthodox Churches still use the older Julian calendar. Because the Julian calendar is currently 13 days behind the Gregorian, the two Easters usually fall on different dates.

The Paschal Full Moon and the Metonic Cycle

The church's calculation of the Paschal Full Moon is based on the 19-year Metonic cycle, which is also a foundational component of the Hebrew calendar. This cycle approximates that 19 solar years are almost exactly equal to 235 lunar months, allowing for a predictable, long-term table of full moon dates without relying on direct observation.