Catechism of the Catholic Church §1193 — excerpt
Sunday, the "Lord's Day," is the principal day for the celebration of the Eucharist because it is the day of the Resurrection. It is the pre-eminent day of the liturgical assembly, the day of the Christian family, and the day of joy and rest from work.
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.