Trinity Sunday is this coming Sunday, 11 June. It falls on the first Sunday after Pentecost, and celebrates the Christian doctrine of the Trinity: one God in three Persons: the Father, the Son and the Holy Spirit.

Theologically, Trinity Sunday acts as the culmination of God's glory being revealed in full: the Resurrection at Easter, the Ascension of Jesus to the Father in Heaven, and the outpouring of the Holy Spirit at Pentecost.

The definition of the Trinity itself developed over time, based on Scripture and tradition. The Scriptures call the Father, the Son, and the Holy Spirit 'God', yet the three are also clearly distinct. For example, St John gives Jesus the titles theos and monogenes theos (God and Only-Begotten God), yet in his Gospel Jesus also states that the Father and Son are two witnesses (John 1:1, 18; 8:17-18; 10:30). The Church had to reconcile the Divinity of Christ and the Holy Spirit with Jewish monotheism, and began using the word Trinity by the middle of the 2nd century to describe the relationship between the Father, Son, and Holy Spirit. The Trinity is emphasised in the Nicene Creed drawn up in AD 325: 'I believe in one God, the Father Almighty...And in one Lord Jesus Christ, the only-begotten Son of God...being of one substance with the Father, by whom all things were made...And I believe in the Holy Ghost, the Lord and Giver of Life; who proceeds from the Father [and the Son]; who with the Father and the Son together is worshipped and glorified; who spoke by the prophets...'

Historically, the origins of Trinity Sunday come from Thomas Becket (1118–70), who was consecrated as Archbishop of Canterbury on the Sunday after Pentecost – Whitsun – and whose first act was to ordain that the day should be held as a new festival in honour of the Holy Trinity. This observance spread from throughout the whole of western Christendom. Trinity Sunday is a principal feast day in the Anglican and is one of seven principal feast days in the Episcopal Church.

The Sundays after Pentecost, until Advent, are numbered as 'Trinity 1, Trinity 2, Trinity 3' and so on.

In the Catholic Church, Trinity Sunday is officially known as the 'Solemnity of the Most Holy Trinity'.