The feast of the Most Holy Trinity is now a Double of the First Class having been raised to that rank in the reforms of 1911-13. Prior to those reforms it was a Double of the Second Class and before that a double. Its origin appears to be as a local feast that originated in Liege in the tenth century with its celebration spreading in northern France and England. The Franciscan John Peckham revised the texts in the thirteenth century. In many local rites (and e.g., in the Dominican rite) Sundays were counted after Trinity rather than Pentecost, as indeed they still are in the BCP. The first Sunday after Pentecost is commemorated in the Office and Mass. The feast of St. Felix is omitted this year.
The singing of first Vespers of the feast yesterday afternoon marked the beginning of the Summer (Pars Aestiva) volume of the Breviarium Romanum. The antiphons Gloria tibi Trinitas etc were sung, doubled, with Pss. 109, 110, 111, 112 and 116. The chapter, O altitudo, and Office hymn, Jam sol recedit, will be used at Vespers on Saturdays for all the Sundays after Pentecost. The antiphon on the Magnificat, Gratias tibi, Deus etc, and the collect were proper to the feast. After the collect of the feast a commemoration of the first Sunday after Pentecost was sung. After Vespers the antiphon Salve Regina etc was sung for the first time this year. At Compline the Dominical preces were omitted.
At Mattins the invitatory is proper, Deum verum, unum in Trinitate, et Trinitatm in Unitate, Venite adoremus. In the first nocturn he antiphons, Adesto, unus Deus etc., are sung with psalms 8, 18 & 23. The lessons are taken from the sixth chapter of the Prophet Isaiah. In the second nocturn the antiphons, Te invocamus etc., are sung with psalms 46, 47 & 71, the lessons are taken from the Book of Bishop Fulgentius on faith. In the third nocturn the antiphons Caritas Pater est etc are sung with psalms 95, 96 & 97. The homily is from St. Gregory Nazianzen. The ninth lesson is of the first Sunday after Pentecost. The Te Deum is sung. At Lauds the antiphons, Gloria tibi, Trinitas etc., are sung with the Dominical psalms (92, 99, 62, Benedicite & 148). The Office hymn is Tu, Trinitatis Unitas. After the collect of the feast a commemoration is sung of the Sunday.
At Prime the festal psalms are sung, 53, 118(i) & 118(ii), under the antiphon Gloria tibi Trinitas. The Creed of St. Athanasius, Quicumque, is sung after the last stanza of Ps. 118. The lectio brevis is Tres sunt.
Mass is sung after Terce. Before Mass at the sprinkling of lustral water the antiphon Asperges me returns. The Mass is proper, Benedicta sit. The Gloria is sung, the second collect is of the Sunday, the Credo is sung, the preface that of the Most Holy Trinity (used for all Sundays not having a proper preface after 1759), and the last Gospel is of the Sunday.
At Vespers the antiphons Gloria tibi, Trinitas etc are sung with the Sunday psalms. After the collect of the feast commemorations are sung of the following feast of St. Angela Merici and of the first Sunday after Pentecost. At Compline the Dominical preces are omitted.
In the 'liturgical books of 1962' so much has been excised from the Breviary that four volumes are no longer needed. The 'Pars Altera' of the two volumes begins today. There is no commemoration of the first Sunday after Pentecost at Vespers, Mattins or Lauds. The eighth lesson is split into two to make a ninth lesson for the feast. At Prime Quicumque is sung on this Sunday alone in the 1962 rite, the lectio brevis is Dominus autem dirigat. At Mass there is no commemoration of the Sunday and therefore no proper last Gospel. At Vespers there are no commemorations.