Computus is the calculation method used to determine the date of Easter Sunday in the Christian tradition.
The date is linked to the first Sunday after the first full moon on or after the spring equinox, although the exact calendars and rules differ among Christian churches. Western churches generally use the Gregorian calendar, while many Eastern churches calculate Easter using the Julian calendar.
Computus developed from early Christian debates about how to date Easter in relation to Passover and the Jewish lunisolar calendar. The First Council of Nicaea in 325 established the principle that Christians should celebrate Easter on a Sunday, but it did not provide every later calculation detail.
Easter therefore moves between dates rather than falling on one fixed day. Computus is not the name of the Easter season or a worship service; it specifically refers to the calendar calculation.