Trinity Sunday
A much later addition to the sequence of the celebrations of Easter was Trinity Sunday. Various commemorations of the Trinity are known of from the late seventh century, but Trinity Sunday acquired its status in the thirteenth century, under pressure from the Cistercians, who observed the feast, and from a desire to counteract the unitarian tendencies of Albigensians and Waldensians. So Trinity Sunday came to be a completion of the cycle that marked the work of Christ and the outpouring of the Spirit. The feast was widely observed in England, and the Book of Common Prayer followed the Sarum tradition of naming the subsequent Sundays as Sundays after Trinity.
For Liturgical Use
Trinity Sunday is unusual in the calendar of the church, in that it marks a piece of theology rather than a person or an event. It developed as a festival in the Middle Ages as a proclamation of God’s threefold action in creating, redeeming and sanctifying us, and as a summation of all the events celebrated in the festivals of God’s saving work in Christ and through the Holy Spirit.
Sentence, Prayers and Readings as in A New Zealand Prayer Book - He Karakia Mihinare o Aotearoa, page 606-607.