Trinity Sunday, officially called "The Solemnity of the Most Holy Trinity," is one of the few celebrations of the Christian Year that commemorates a reality and doctrine rather than a person or event. On Trinity Sunday, Christians remember and honor the eternal God: the Father, the Son, and the Holy Spirit. Trinity Sunday is celebrated the Sunday after Pentecost, and lasts only one day, which is symbolic of the unity of the Trinity. Eastern Churches have no tradition of Trinity Sunday, arguing that they celebrate the Trinity every Sunday. Westerners also commemorate the Trinity at every Eucharist, although they set aside a special feast day for the purpose as well.
Trinity Sunday is celebrated in all the Western liturgical churches: Roman Catholic, Anglican, Lutheran, Presbyterian, and Methodist.
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 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.
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 Afterfeas.
