This title may be simplifying things a bit, but while the Blessed and Most Holy Trinity is pure Mystery, while reflecting on this divine reality, that is what I thought, that the Solemnity of the Trinity that we celebrate this Sunday is truly God's feast day. It celebrates God~Father, Son and Holy Spirit for the Godhead they are. There are no words to explain or describe this adequately. I guess in my feeble attempt, what I mean by all this is that today we celebrate God for being God.
As far as my thoughts go, I will leave it at that. However, there have been saints who in their earthly lives have been given the grace of a deeper knowledge or understanding of the Trinity. St Faustina Kowalska is one of them. Here are a few excerpts from her Diary on the subject of the Trinity in her own life.
I understand the spiritual espousal of a soul with God, which has no exterior manifestation. It is a purely interior act between the soul and God. This grace has drawn me into the very burning center of God's love. I have come to understand His Trinitarian quality and the absolute Oneness of His Being. (Diary 1020)
After Holy Communion I communed for a while with the heavenly Father. My soul was drawn into the glowing center of love. I understood that no exterior works could stand comparison with the pure love of God. ... I saw the joy of the Incarnate Word, and I was immersed in the Divine Trinity. When I came to myself, longing filled my soul, and I yearned to be united with God (Diary, 1121).
And I will close with the TeDeum and by wishing all fathers a Happy Father's Day.
WE praise thee, O God; we acknowledge thee to be the Lord.
All the earth doth worship thee, the Father everlasting.
To thee all Angels cry aloud; the Heavens, and all the Powers therein;
To thee Cherubim and Seraphim continually do cry,
Holy, Holy, Holy, Lord God of Sabaoth;
Heaven and earth are full of the Majesty of thy glory.
The glorious company of the Apostles praise thee.
The goodly fellowship of the Prophets praise thee.
The noble army of Martyrs praise thee.
The holy Church throughout all the world doth acknowledge thee;
The Father, of an infinite Majesty;
Thine adorable, true, and only Son;
Also the Holy Ghost, the Comforter.
THOU art the King of Glory, O Christ.
Thou art the everlasting Son of the Father.
When thou tookest upon thee to deliver man, thou didst humble thyself to be born of a Virgin.
When thou hadst overcome the sharpness of death, thou didst open the Kingdom of Heaven to all believers.
Thou sittest at the right hand of God, in the glory of the Father.
We believe that thou shalt come to be our Judge.
We therefore pray thee, help thy servants, whom thou
hast redeemed with thy precious blood.
Make them to be numbered with thy Saints, in glory everlasting.
O LORD, save thy people, and bless thine heritage.
Govern them, and lift them up for ever.
Day by day we magnify thee;
And we worship thy Name ever, world without end.
Vouchsafe, O Lord, to keep us this day without sin.
O Lord, have mercy upon us, have mercy upon us.
O Lord, let thy mercy be upon us, as our trust is in thee.
O Lord, in thee have I trusted; let me never be confounded.