Catechism of the Catholic Church §1165 — excerpt
When the Church celebrates the mystery of Christ, there is a word that marks her prayer: "Today!" - a word echoing the prayer her Lord taught her and the call of the Holy Spirit.
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.
Also cites: St. Hippolytus, De pasch. 1-2 SCh 27, 117