Catechism of the Catholic Church §1387 — excerpt
To prepare for worthy reception of this sacrament, the faithful should observe the fast required in their Church. Bodily demeanor (gestures, clothing) ought to convey the respect, solemnity, and joy of this moment when Christ becomes our guest.
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. 919