Trinity Sunday was first celebrated by Bishop Stephen of Liège in the early 10th century and by the twelfth century the custom had spread to France where it was observed at Cluny Abbey.
Archbishop Thomas Becket introduced it to England having been consecrated and as Archbishop at Canterbury Cathedral on the Sunday after Pentecost (or Trinity Sunday) 1162.
Appropriately his shrine was to be placed in the Trinity Chapel of Canterbury Cathedral.
In 1334 Pope John XXII added Trinity Sunday to the universal calendar along with next Thursday’s Corpus Christi which also originated in Liège.
The Trinity is God the Father, Son and Holy Spirit, all in one, as alluded to at Christ’s baptism when the Spirit descended as a dove (Mark 1: 9-11).
Today we are sent out from church Sunday by Sunday (maybe online) in the name of the Father, Son and Holy Spirit.
Like St Paul we say in prayer may The grace of our Lord Jesus Christ, the love of God and the fellowship of the Holy Spirit be with us all (2 Corinthians 13: 13).