Catechism of the Catholic Church §1170 — excerpt
At the Council of Nicaea in 325, all the Churches agreed that Easter, the Christian Passover, should be celebrated on the Sunday following the first full moon (14 Nisan) after the vernal equinox.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.