Catechism of the Catholic Church §1104 — excerpt
Christian liturgy not only recalls the events that saved us but actualizes them, makes them present. the Paschal mystery of Christ is celebrated, not repeated.
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.