Trinity Sunday is the feast of the Holy Trinity — the one God in three Persons, Father, Son, and Holy Spirit. It falls on the first Sunday after Pentecost, completing the great arc of the liturgical calendar: the Father sends the Son (Christmas), the Son accomplishes redemption (Easter), the Spirit is given to the Church (Pentecost), and on Trinity Sunday the Church confesses the fullness of God as revealed through these mighty acts. Unlike most feasts, Trinity Sunday commemorates not an event but a doctrine — the central mystery of the Christian faith. The feast is comparatively late in origin. While Trinitarian theology was definitively settled at Nicaea (325) and Constantinople (381), a distinct feast of the Trinity did not become widespread until the medieval period. Individual bishops promoted it — Thomas Becket, consecrated on Trinity Sunday 1162, established it at Canterbury. Pope John XXII made it universal for the Western church in 1334. The BCP tradition, however, gave Trinity Sunday a structural importance beyond Rome's: in the English Prayer Book tradition, the Sundays of the long season after Pentecost are numbered 'after Trinity' rather than 'after Pentecost,' making Trinity Sunday the hinge of the entire second half of the church year. The BCP 2019 lists it among the seven Principal Feasts, and the Athanasian Creed — the most detailed creedal exposition of Trinitarian doctrine — is traditionally appointed for this day.

Trinity Sunday's liturgical tradition reflects the challenge of celebrating a doctrine rather than a narrative. The appointed readings across the three-year lectionary cycle present different facets of Trinitarian revelation: Nicodemus's nighttime conversation with Jesus (John 3), the baptismal commission 'in the name of the Father and of the Son and of the Holy Spirit' (Matthew 28), and Paul's Trinitarian benediction (2 Corinthians 13:14). The Athanasian Creed (Quicunque Vult), with its precise and paradoxical formulations — 'neither confounding the Persons nor dividing the Substance' — has been associated with this feast since at least the ninth century in the West, and the BCP 2019 includes it among its historical documents. The collect for Trinity Sunday is among the most theologically careful in the Prayer Book: 'by the confession of a true faith, to acknowledge the glory of the eternal Trinity, and in the power of your divine Majesty to worship the Unity.' Cranmer's 1549 language echoes here. The white or gold vestments used on this day reflect its status as a feast of the Godhead. In Anglican practice, numbering Sundays 'after Trinity' rather than 'after Pentecost' has given this feast a unique structural role — it names half the church year.

Dunstan was born in 909 near Glastonbury into a family connected to the West Saxon royal house. He was educated at Glastonbury Abbey and showed early talent for music, metalwork, and illumination — arts he practiced throughout his life. It is from this work at the forge that the later medieval tradition draws his most famous image: that while making liturgical vessels he was tempted by the devil and seized him by the nose with red-hot tongs. The episode is unknown to his earliest biographer ('B.,' writing in the 990s) and first appears in Osbern of Canterbury more than a century later; it nevertheless became the iconographic signature by which Dunstan was known throughout the English Middle Ages.

His early career was politically turbulent. He was a courtier under King Æthelstan but was expelled from court on accusations of practicing black magic (likely the suspicion of a jealous rival). After a period of uncertainty, King Edmund appointed him Abbot of Glastonbury around 940. Under Dunstan's leadership, Glastonbury became the model for the English Benedictine revival: he enforced strict observance of the Rule, built up the library, promoted the arts, and trained a generation of monks who would become bishops and reformers.

Dunstan served as chief adviser to successive kings — Edmund, Eadred, and Edgar. Under Edgar (959–975), the monastic reform reached its peak. Dunstan, together with his fellow reformers Æthelwold (Bishop of Winchester) and Oswald (Bishop of Worcester, later Archbishop of York), reorganized English monasticism along Continental Benedictine lines. The Regularis Concordia, a unified monastic customary drawn up at the Council of Winchester (c. 970), codified their reforms.

As Archbishop of Canterbury (959–988), Dunstan wielded enormous ecclesiastical and political power. He crowned Edgar in an elaborate ceremony at Bath in 973 — a liturgical event that established the pattern for English coronations thereafter. After Edgar's death, Dunstan navigated the difficult succession between Edward the Martyr and Æthelred the Unready.

Dunstan died on May 19, 988, in his eightieth year. His last recorded public act was preaching three sermons on Ascension Day, during which he is said to have announced his imminent death.

The most famous legend about Dunstan tells that one night, while he was at work in his forge, the devil appeared at the window in human disguise — in some tellings as a beautiful woman — and sought to draw him into temptation. Dunstan, recognizing his adversary, snatched up the red-hot tongs from the fire and seized the devil by the nose, holding him fast as he howled and writhed; the cries (a later English rhyme adds) were heard three miles off before he was at last released. The story connects to Dunstan's known skill as a metalworker and became his defining iconographic attribute. It first appears, however, in Osbern of Canterbury's Vita (c. 1090), over a century after Dunstan's death; it has no early attestation and belongs to the classic devil-encounter hagiography of the post-Conquest period.

The earliest vita ('B.') records that Dunstan experienced visions and that his appointment to Glastonbury was divinely confirmed, but these are relatively restrained compared to the later tradition. 'B.' also records miracles at Dunstan's tomb, following standard convention.

Dunstan's prediction of his own death on Ascension Day is reported by the earliest sources and has the character of a deathbed prophecy topos, though the specific liturgical context (three sermons on the feast) gives it circumstantial weight.