Feast Of The Most Holy Trinity

"Glory be to the Father, and to the Son, and to the Holy Ghost.

As it was in the beginning, is now, and ever shall be, world without end. Amen."

As soon as we have celebrated the Advent of the Holy Ghost, we celebrate the Feast of the Holy Trinity in the office of the following Sunday.

The time is well chosen, for, immediately after the descent of this Divine Spirit, began the preaching and belief and Baptism and confession in the Name of the Father, the Son and the Holy Ghost. (St. Rupert)
The Roman Missal, 1962

Introit (Cf. Tobias 12.6)
Blessed be the holy Trinity, and undivided Unity: we will give glory to Him, because He hath shown His mercy to us. Ps. 8. 2. O Lord, our Lord, how wonderful is Thy Name in all the earth! Glory be to the Father, and to the Son, and to the Holy Ghost. Blessed be the holy Trinity, and undivided Unity: we will give glory to Him, because He hath shown His mercy to us.

Collect
O Almighty and everlasting God, who hast enabled Thy servants, in confessing the true Faith, to acknowledge the glory of the eternal Trinity, and in the power of Majesty to adore its Unity: we beseech Thee, that by steadfastness in the same Faith, we may ever be defended against all adversity. Through our Lord Jesus Christ, Thy Son, who liveth and reigneth with Thee in the unity of thy Holy Ghost, One God, now and forever. Amen