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 Pope Pius V had removed. See, below, "Some differences in relation to later editions of the Roman calendar".
Pius V removed from the existing Roman calendar many mediaeval saints, keeping only about half a dozen who had been canonized after the eleventh century. His calendar did not include Saints Joachim, Anne, Anthony of Padua, Nicholas of Tolentino, Francis of Paola, Bernardino of Siena or Elizabeth of Hungary, nor any anatomical feasts, such as that of the Stigmata of Saint Francis of Assisi, or the Precious Blood or the Five Wounds of Christ. He removed the word "Immaculate" from the title of the 8 December feast of the Immaculate Conception of the Blessed Virgin Mary, abolished the previously existing special Mass for that day, whose Introit and Collect would be restored by Pope Pius IX, and directed that the Mass of the Nativity of Mary should be used instead, but with the word "Conception" (not "Immaculate Conception") replacing the word "Nativity" when used on 8 December. He raised to the rank of double the feasts of the four Eastern Saints Athanasius of Alexandria, Basil of Caesarea, John Chrysostom and Gregory of Nazianzus, and, while he did not give them the title of Doctor of the Church, he assigned to them the common used for the four Western Doctors, Pope Gregory I, Augustine of Hippo, Jerome and Ambrose. On the other hand, he lowered the ranks of many saints' feasts, in order to allow celebration of Sundays and the ferias of Advent and Lent, for any double feast outranked an ordinary Sunday of the year until St. Pius X (see Reform of the Roman Breviary by Pope Pius X), and it was not until the reforms of John XXIII that ferias of Lent and, from 17 to 23 December, those of Advent outranked third-class feasts (which included most of the feasts formerly of Double rank).
In the Tridentine Calendar, the rank of feasts is expressly indicated only if they are ranked as Double or Semidouble, while absence of an indication means that a feast is of the rank of Simple. (For the meaning of these terms see Ranking of liturgical days in the Roman Rite.) This tripartite ranking as Double, Semidouble, and Simple originated in the thirteenth century and, apart from deciding precedence in the case of two celebrations coinciding on the same day (as when a feast of the fixed calendar coincided with a Sunday, or with a feast or octave whose date depended on that of Easter), was of practical importance more for the Liturgy of the Hours than for the Mass.
Pope Clement VIII introduced the rank of Major Double in 1602. This distinction and those of Double of the First Class and Double of the Second Class are absent in the Tridentine Calendar.
While St Pius V reduced the number of feast days, later Popes repeatedly added more and altered the ranking of already existing feasts. Even the 1969 revision of the General Roman Calendar, which kept significantly fewer feasts than before, still had more than Pius V's Tridentine Calendar. The Catholic Encyclopedia published the following chart to document the incremental growth of saints' days down to 1907.
Soon after the publication of this 1907 table, Pope Pius X made a general revision of the rubrics of the calendar, the result of which (with a few additions by Pope Pius XI) can be seen in General Roman Calendar of 1954. This was followed by Pope Pius XII's simplifying revision of 1955 (see General Roman Calendar of Pope Pius XII). Pope John XXIII's General Roman Calendar of 1960 reduced the number of celebrations and completely abandoned the ranking as Doubles, Simples, etc. This calendar is still authorized for use in accordance with the motu proprio Summorum Pontificum of Pope Benedict XVI, which states that the Tridentine Mass was never abrogated. For the 1969 revision, which with subsequent adjustments is in general use in the Latin Church (the present General Roman Calendar, observed for instance by the Pope himself) see General Roman Calendar of 1969.
In leap years, a day is added and it is of 29 days but the Feast of St. Matthias is celebrated on the 25th day and then is said twice Sexto Kalendas, that is on the 24th and 25th day, and thus the Dominical letter is changed to the one above, that if it be B, into A, if it be C, into B, similarly also in the others.
The Octaves (plural) mentioned for the last days of December are those of the Nativity, of St Stephen, of St John, and of the Holy Innocents.
Although not listed on the General Calendar, a commemoration of St Anastasia martyr is made at the second Mass on 25 December (pages 22–23 of the Ordinarium Missarum de tempore section of the Tridentine Roman Missal), and commemorations are made of St John and the Holy Innocents on 2 January; the Octave of St Stephen, and of the Holy Innocents on 3 January; the Octave of St John (page 40 of the same section of the Missal). In addition, on every feast of St Peter there is a commemoration of St Paul and on every feast of St Paul a commemoration of St Peter (page 10 of the Proprium Missarum de Sanctis section of the Missal).