III. When is the Liturgy Celebrated?

From The Catechism of the Catholic Church

1170
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.

PART TWO: THE CELEBRATION OF THE CHRISTIAN MYSTERY

SECTION ONE THE SACRAMENTAL ECONOMY
- CHAPTER TWO THE SACRAMENTAL CELEBRATION OF THE PASCHAL MYSTERY
  - Article 1 CELEBRATING THE CHURCH’S LITURGY
    - III. When is the Liturgy Celebrated?

From The Catechism of the Catholic Church – rosary.team