The now-popular Feast of the Holy Trinity developed relatively slowly and was not established as an official Feast of the Church until the fourteenth century. It is unique among the festivals of the Church Year, in that it does not celebrate an event or a person from the history of salvation, but revels in the Self-revelation and character of the one true God Himself — not simply the doctrine of the Holy Trinity, but the Holy Triune God Himself and His mercy upon us. In the words of the Antiphon: “Blessèd be the Holy Trinity and the undivided Unity. Let us give glory to Him because He has shown His mercy to us” (LSB Altar Book, 906).
Of particular significance is the way the Holy Trinity is confessed and celebrated in the Church’s worship of the Father, Son, and Holy Spirit. That point is already evident in the way the Nicene Creed confesses the Holy Spirit, “who with the Father and the Son together is worshiped and glorified.” But the point is made explicit in several ways on the Feast of the Holy Trinity. In the Collect of the Day, for example, we acknowledge that God has “given us grace to acknowledge the glory of the eternal Trinity by the confession of a true faith and to worship the Unity in the power of the Divine Majesty” (LSB Altar Book, 906). Likewise, in the Proper Preface for Holy Trinity, “in the confession of the only true God, we worship the Trinity in Person and the Unity in Substance, of Majesty coequal” (LSB Altar Book, 235). Similarly, a beautiful hymn of the Holy Trinity, “Father Most Holy” (LSB 504), affirms and confesses the following: “Maker of all things, all Thy creatures praise Thee; all for Thy worship were and are created; now, as we also worship Thee devoutly, hear Thou our voices” (stanza 3).
In each case, the focus of the Feast of the Holy Trinity is not on doctrine per se, but on the Mystery and adoration of the Triune God Himself, as articulated in the Athanasian Creed (customarily confessed on this festival day): “The catholic faith is this, that we worship one God in Trinity and Trinity in Unity” (LSB 319).
Although the Feast of the Holy Trinity is the Octave of Pentecost Day, it returns to the color white in honor of the Father, Son, and Holy Spirit. It thereby recalls the festive adornment of Eastertide, and it is distinguished from the long series of green Sundays that follow throughout the Time of the Church. The strong baptismal emphases in many of the Readings appointed for Holy Trinity also connect this Feast to the liturgical focus of Easter and Pentecost (St. Matthew 28:16–20 and St. John 3:1–16, in particular).
The relationship of Holy Trinity to Pentecost Day, as well as the transitional character of these festival days, is evident in the intended three-year series Gradual for the Feast of the Holy Trinity and for the Sundays following Holy Trinity (Propers 3–7, as needs may be, from 24 May through 25 June). (The Graduals intended for the Time of the Church in all three years of the lectionary are those actually included in Series C. Regrettably, the Graduals indicated in Series A and B were inadvertently retained “as is” from LW.) The first portion of the intended Gradual (Romans 10:8b) emphasizes the preaching of the Word of faith, constitutive of the Lord’s Church. The second portion of this Gradual (Romans 10:10) remains the same as the one appointed for Pentecost Day, highlighting faith in the heart and the confession of that faith: “The Word is near you, in your mouth and in your heart, the Word of faith that we proclaim. With the heart one believes and is justified, and with the mouth one confesses and is saved” (LSB Altar Book, 805).