The Tridentine calendar is the calendar of saints to be honoured in the course of the liturgical year in the official liturgy of the Roman Rite as reformed by Pope Pius V, implementing a decision of the Council of Trent, which entrusted the task to the Pope.
Use of both these texts, which included Pius V's revised calendar, was made obligatory throughout the Latin Rite except where other texts of at least two centuries' antiquity were in use, and departures from it were not allowed. The Apostolic Constitution Quod a nobis, which imposed use of the Tridentine Roman Breviary, and the corresponding Apostolic Constitution Quo primum concerning the Tridentine Roman Missal both decreed: "No one whosoever is permitted to alter this letter or heedlessly to venture to go contrary to this notice of Our permission, statute, ordinance, command, precept, grant, indult, declaration, will, decree and prohibition. Should anyone, however, presume to commit such an act, he should know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul." See the article on Quo primum.
Pius V himself altered his Calendar when, after the victory in 1571 of the battle of Lepanto, he added the feast of Our Lady of Victory. In 1585, Pope Sixtus V restored the feast of the Presentation of the Virgin Mary, which Pius V had removed. See, below, "Some differences in relation to later editions of the Roman calendar".