Nicaea ruled in 325 that the whole church keep Easter on one Sunday and stop taking its date from the Jewish calendar — but it fixed no arithmetic, and someone had to compute, every year and in advance, the Sunday after the first full moon on or after the equinox. Alexandria kept the reckoning and announced the date to the rest by festal letter; the Latin West disagreed with it for two centuries. Dionysius Exiguus put the Alexandrian cycle into Latin in 525, and that table is what the West eventually took. The date is computed, not observed, which is why it still wanders across five weeks.
The Seven Ecumenical Councils (NPNF series 2 vol. 14) · NPNF2 XIV — Nicaea I, the synodal letter to the Alexandrians