Catechism of the Catholic Church §2177 — excerpt
The Sunday celebration of the Lord's Day and his Eucharist is at the heart of the Church's life. "Sunday is the day on which the paschal mystery is celebrated in light of the apostolic tradition and is to be observed as the foremost holy day of obligation in the universal Church."
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: CIC, can. 1246 # 1 · CIC, can. 1246 # 2: "The conference of bishops can abolish certain holy days of obligation or transfer them to a Sunday with prior approval of the Apostolic See."