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|Observed by||Western Christianity|
|Date||Sunday after Pentecost|
|Related to||Christmas, Holy Week, Easter, Ascension Thursday and Pentecost which precede it and Corpus Christi and the beginning of the second portion of Ordinary Time which follow it|
Trinity Sunday is the first Sunday after Pentecost in the Western Christian liturgical calendar, and the Sunday of Pentecost in Eastern Christianity. Trinity Sunday celebrates the Christian doctrine of the Trinity, the three Persons of God: the Father, the Son, and the Holy Spirit.
Western Christianity 
Church year 
The Sundays following Pentecost, until Advent, are numbered from this day. In traditional Catholic usage, the First Sunday After Pentecost is on the same day as Trinity Sunday. In the revised Roman rite, Ordinary Time resumes one week earlier, on the Monday after Pentecost, with the Sundays that would otherwise fall on Pentecost and Trinity Sunday omitted that year. In the Church of England, following the pre-Reformation Sarum use, the following Sunday is the "First Sunday after Trinity", while the Episcopal Church in the United States of America (ECUSA) now follows the Catholic usage, calling it the Second Sunday after Pentecost. The liturgical colour used on Trinity Sunday is white.
In the Catholic Church it is officially known as the Solemnity of the Most Holy Trinity. Prior to the reforms of the Second Vatican Council, it marked the end of a three-week period when church weddings were forbidden. The period began on Rogation Sunday, the fifth Sunday after Easter. Trinity Sunday was established as a Double of the Second Class by Pope John XXII to celebrate the Trinity. It was raised to the dignity of a Double of the First Class by Pope Pius X on 24 July 1911.
In the traditional Divine Office, the Athanasian Creed (Quicumque vult) is said on this day at Prime. Before 1960, it was said on all Sundays after Epiphany and Pentecost which do not fall within Octaves or on which a feast of Double rank or higher was celebrated or commemorated, as well as on Trinity Sunday. The 1960 reforms reduced it to once a year, on this Sunday.
In the 1962 Missal, the Mass for the First Sunday After Pentecost is not said or commemorated on Sunday (it is permanently impeded there by Trinity Sunday), but is used during the week if the ferial Mass is being said.
The Thursday after Trinity Sunday is observed as the Feast of Corpus Christi. In some countries, including the United States, Canada, and Spain, it may be celebrated on the following Sunday, when parishioners are more likely to attend Mass and be able to celebrate the feast.
The Creed of Saint Athanasius, although not often used, is recited in certain Anglican churches, particularly those of High Church tendency. It may be found in the Historical Documents section of the 1979 Book of Common Prayer (Episcopal Church), but its use is not specifically provided for in the rubrics of that prayer book.
Thomas Becket (1118–70) was consecrated Archbishop of Canterbury on the Sunday after Whit Sunday, and his first act was to ordain that the day of his consecration should be held as a new festival in honour of the Holy Trinity. This observance spread from Canterbury throughout the whole of Christendom.
Anglo-Catholic parishes observe Corpus Christi on the following Thursday, or in some cases the following Sunday.
Trinity Sunday is the Sunday following Pentecost, and eight weeks after Easter Sunday. Following are the dates of Trinity Sunday in the Western churches (for the dates of Trinity Sunday in the Eastern Churches, see Pentecost):
- 2013: 26 May
- 2014: 15 June
- 2015: 31 May
- 2016: 22 May
- 2017: 11 June
- 2018: 27 May
- 2019: 16 June
- 2020: 7 June
The earliest possible date is 17 May, as in 1818 and 2285. The latest possible date is 20 June, as in 1943 and 2038.
In the early Church, no special Office or day was assigned for the Holy Trinity. When the Arian heresy was spreading, the Fathers prepared an Office with canticles, responses, a Preface, and hymns, to be recited on Sundays. In the Sacramentary of St. Gregory the Great (P.L., LXXVIII, 116) there are prayers and the Preface of the Trinity. The Micrologies (P.L., CLI, 1020), written during the pontificate of Gregory VII (Nilles, II, 460), call the Sunday after Pentecost a Dominica vacans, with no special Office, but add that in some places they recited the Office of the Holy Trinity composed by Bishop Stephen of Liège (903-20). By others the Office was said on the Sunday before Advent. Alexander II (1061–1073), refused a petition for a special feast on the plea, that such a feast was not customary in the Roman Church which daily honoured the Holy Trinity by the Gloria Patri, etc., but he did not forbid the celebration where it already existed. John XXII (1316–1334) ordered the feast for the entire Church on the first Sunday after Pentecost. A new Office had been made by the Franciscan John Peckham, Canon of Lyons, later Archbishop of Canterbury (d. 1292). The feast ranked as a double of the second class but was raised to the dignity of a primary of the first class, 24 July 1911, by Pius X (Acta Ap. Sedis, III, 351). Since it was after the first great Pentecost that the doctrine of the Trinity was proclaimed to the world, the feast becomingly follows that of Pentecost.
Eastern Christianity 
In the Eastern Orthodox and Eastern Catholic churches, the Sunday of Pentecost itself is called Trinity Sunday (the Sunday after Pentecost is All Saints Sunday). The Monday after Pentecost is called Monday of the Holy Spirit, and the next day is called the Third Day of the Trinity. Though liturgical colours are not as fixed in the Eastern practice (normally there are simply "festive" colours and "somber" or Lenten colours), in some churches, green is used for Pentecost and its Afterfeast.
Bach cantatas 
- Anniversaries and holidays by Bernard Trawicky, Ruth Wilhelme Gregory 2000 ISBN 0-8389-0695-8 page 225