Easter, Easter Sunday (Western)

Easter's date rests on a rule rather than a fixed day, the first Sunday after the first full moon following the spring equinox, a formula the Council of Nicaea in 325 CE sought to fix as common practice across a Christian world that had until then followed several different local methods. Nicaea did not achieve immediate uniformity even so, and Alexandria and Rome continued computing the date by different tables for some generations afterward before Western practice settled on a shared computus.

What reopened the split permanently was the Gregorian calendar reform of 1582, which corrected the accumulated drift in the Julian calendar's own reckoning of the equinox. The West adopted the new calendar and recalculated Easter against it, while Eastern Orthodox churches kept computing the date using the older Julian equinox and lunar tables, an approach kept separately under the Orthodox reckoning. The result is a single ancient formula applied to two different calendars, producing dates that coincide in some years and fall as much as five weeks apart in others, a divergence that traces to the sixteenth century calendar reform rather than to any disagreement over what Easter itself commemorates.