Roman Catechism 18.168

Chapter 18 — Part II: The Sacraments — The Holy Eucharist

Section 168. The Eucharist as a Sacrifice — We must now proceed to explain its nature as a Sacrifice, that pastors may understand what are the principal instructions which they ought to impart to the faithful on Sundays and holy days, regarding this mystery in conformity with the decree of the holy Council (of Trent).