Feast of the Most Holy Trinity
On the day of Pentecost the holy Apostles received, as we have seen, the grace of the Holy Ghost. In accordance with the injunction of their Divine Master, (Matthew 28:19) they will soon start on their mission of teaching all nations and baptizing men in the name of the Holy Trinity. It was but right, then, that the solemnity which is intended to honor the mystery of one God in three Persons should immediately follow that of Pentecost, with which it has a mysterious connection. And yet, it was not until after many centuries that it was inserted in the cycle of the liturgical year, whose completion is the work of successive ages. Every homage paid to God by the Church's liturgy has the holy Trinity as its object. Time, as well as eternity, belongs to the Trinity. The Trinity is the scope of all religion. Every day, every hour, belongs to It. The feasts instituted in memory of the mysteries of our redemption center in It. The feasts of the blessed Virgin and the Saints are but so many means for leading us to the praise of the God who is One in essence, and Three in Persons. This Sunday's office, in a very special way, gives us, each week a most explicit expression of adoration and worship of this mystery, which is the foundation of all others, and the source of all grace. This explains to us how it is that the Church was so long in instituting a special feast in honor of the Holy Trinity. The ordinary motive for the institution of feasts did not exist in this instance. A feast is the memorial of some fact which took place at a certain time, and of which it is well to perpetuate the remembrance and the influence. How could this be applied to the mystery of the Trinity? From all eternity, before any created being existed, God liveth and reigneth, Father, Son, and Holy Ghost. If a feast in honor of that mystery were to be instituted, it could only be by fixing some one day in the year, whereon the faithful would assemble for offering a more than usually solemn tribute of worship to the mystery of Unity and Trinity in the one same Divine Nature.
Gueranger, Dom Prosper. The Liturgical Year: Volume X - Time After Pentecost - Book One (pp. 9-10). Kindle Edition.