The Catholic Church celebrates this unfathomable mystery each year on the Sunday following Pentecost. The Catechism of the Catholic Church (paragraph 234) teaches, The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in Himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the “hierarchy of the truths of faith.” https://www.ewtn.com/catholicism/seasons-and-feast-days/holy-trinity-sunday-21087