Today is Trinity Sunday.
The first Sunday after Pentecost, instituted to honour the Most Holy Trinity. 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 other the Office was said on the Sunday before Advent. Alexander II (1061-1073), not III (Nilles, 1. c.), 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). The Greeks have no special feast. 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.
Also, from the Catholic Encyclopedia on the Holy Trinity
The Trinity is the term employed to signify the central doctrine of the Christian religion -- the truth that in the unity of the Godhead there are Three Persons, the Father, the Son, and the Holy Spirit, these Three Persons being truly distinct one from another.
Thus, in the words of the Athanasian Creed: "the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God." In this Trinity of Persons the Son is begotten of the Father by an eternal generation, and the Holy Spirit proceeds by an eternal procession from the Father and the Son. Yet, notwithstanding this difference as to origin, the Persons are co-eternal and co-equal: all alike are uncreated and omnipotent. This, the Church teaches, is the revelation regarding God's nature which Jesus Christ, the Son of God, came upon earth to deliver to the world: and which she proposes to man as the foundation of her whole dogmatic system.
I can't help but think about a certain priest on this day. Fr. Val Rykowski who passed away early this spring was particularly devoted to the Holy Trinity. He is seen, with Bishop Athanasius Schneider of Karaganda, Kazakhstan, when he visited metro-Detroit last year.
- Catechism of the Catholic Church on the Trinity
- Fr. John Hardon on the Trinity (including discussion of heresies regarding the Trinity)
- St. Augustine on the Trinity (Catholic Encyclopedia)