Catechism of the Catholic Church §2191 — excerpt
The Church celebrates the day of Christ's Resurrection on the "eighth day," Sunday, which is rightly called the Lord's Day (cf SC 106).
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.