The liturgical year
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. The reform of the Western calendar, called "Gregorian" after Pope Gregory XIII (1582), caused a discrepancy of several days with the Eastern calendar. Today, the Western and Eastern Churches are seeking an agreement in order once again to celebrate the day of the Lord's Resurrection on a common date.
Source: Catechism of the Catholic Church (Vatican.va)