The Solemnity of the Most Holy Trinity is celebrated on the Sunday following Pentecost Sunday.
The Most Holy Trinity is beyond the understanding of mere man, though many have tried to explain it.
St Patrick used the three-leaf clover to explain the Trinity - comprising three separate leaves but all together being only one plant.
The Blessed Trinity is of course, Father, Son and Holy Spirit - all distinct "persons" yet all comprising the one God.
According to the Encyclopedic Dictionary of the Bible, The Father is unbegotten, the Son begotten, and the Holy Spirit proceeds from the Father and Son as from one principle.
The Blessed Trinity is a mystery - even learned theologians cannot understand it, so we common people are also in the same boat. But we believe that there is but one God composed of three "persons", Father Son and Holy Spirit. And on this day we celebrate that.
As human beings we could not have worked out for ourselves the existence of this Blessed Trinity - we had to be told about it.
The mystery of the Trinity is simply foreshadowed in the Old Testament, but explicitly proclaimed in the New Testament.
Jesus, the Son, makes mention of both the Father and the Spirit in various places. See Mt. 10:20; Gal 4:6; John 16,13-15, and Genesis 1.26. From these and other similar references the Church determined that God consists of the three equal "persons", bound together by love, since God is love.
In 2 Corinthians, 13:13-14 is found the beautiful expression -"All the saints greet you. The grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all."
Pope John XXII established this Feast Day for universal observance in the Western Church, in AD 1334,