Ascension Day commemorates Christ's bodily ascension into heaven forty days after the Resurrection (Acts 1:1-11, Luke 24:50-53). The feast always falls on a Thursday — forty days counted inclusively from Easter Sunday. The observance is ancient. Augustine of Hippo (Ep. 54 to Januarius, c. 400) already regarded the Ascension as a feast of apostolic origin, placing it alongside Easter and Pentecost as celebrations 'which the whole Church observes throughout the world.' Eusebius of Caesarea mentions it in the early fourth century. The Ascension stands as the christological complement to the Incarnation: at Christmas, God descends to take human nature; at the Ascension, human nature is taken up into the life of God. The theological implications are enormous — our flesh, our humanity, is now at the right hand of the Father. The BCP 2019 lists Ascension among the seven Principal Feasts, giving it the highest possible liturgical precedence. The ten days between Ascension and Pentecost have traditionally been kept as days of prayer and expectation, recalling the disciples' waiting in the upper room (Acts 1:12-14). Some Anglican and Roman churches transfer the observance to the following Sunday, though the BCP 2019 does not explicitly authorize this practice.

The Ascension has shaped both the liturgical calendar and Christian architecture. The tradition of extinguishing the Paschal Candle after the Gospel reading on Ascension Day — symbolizing Christ's departure from visible presence — was widespread in the medieval West, though modern practice varies. Egeria records a vigil service at the Imbomon (the traditional site of the Ascension on the Mount of Olives) in the 380s, and the feast was well established in both East and West by the fifth century. In the Byzantine tradition, the Ascension is one of the Twelve Great Feasts, and its icon — Christ ascending in a mandorla of glory while the apostles gaze upward and angels announce his return — is among the most theologically rich in the Eastern iconographic tradition. The two men in white (Acts 1:10-11) who promise Christ's return connect the Ascension forward to the Second Coming, making it both a feast of accomplished salvation and eschatological hope. The collect in the BCP 2019 focuses on the ascent of hearts and minds: 'May our hearts and minds also there ascend, and with him continually dwell.' The season between Ascension and Pentecost — sometimes called the 'Great Novena' — is a period of prayer for the coming of the Spirit.

Almost nothing is known of Augustine's life before his mission. He was prior of the monastery of St. Andrew on the Coelian Hill in Rome when Pope Gregory the Great selected him to lead a missionary expedition to Britain in 596. Gregory had long desired to evangelize the Anglo-Saxons — Bede records the tradition that Gregory, seeing fair-haired English slaves in the Roman market, declared them 'not Angles but angels' (non Angli sed Angeli).

The mission nearly collapsed before it began. The monks, intimidated by reports of the ferocity of the Anglo-Saxons and the dangers of the journey, persuaded Augustine to return to Rome and beg release. Gregory refused, sent Augustine back with letters of encouragement and formal authority, and the party finally crossed the Channel in 597, landing on the Isle of Thanet in Kent.

King Æthelberht, though still pagan, received them cautiously — insisting on meeting outdoors, where he believed magic would have less power. Impressed by their sincerity, he granted them a residence in Canterbury and permission to preach. Within a year Æthelberht himself was baptized, and mass conversions followed.

Augustine established his cathedral at Canterbury and was consecrated archbishop by Gallic bishops, receiving the pallium from Gregory. He founded the monastery of Saints Peter and Paul (later St. Augustine's) and consecrated Mellitus as Bishop of London and Justus as Bishop of Rochester.

His efforts to bring the existing British Christian communities into unity with Canterbury were less successful. A conference with British bishops at a location Bede calls 'Augustine's Oak' broke down over disputes about Easter dating and other customs. Augustine died around May 26, 604 or 605.

Traditionally, Augustine's mission was confirmed by miraculous signs. According to Bede, drawing on earlier Canterbury sources, Augustine performed healings that convinced the pagan English of the power of the Christian God. Gregory, writing to Augustine in a letter preserved by Bede (HE I.31), counseled him not to take pride in miracles but to rejoice that his converts' names were written in heaven — a caution that itself confirms miracle claims were circulating during Augustine's lifetime.

Bede also records a story — which he presents cautiously — that when British bishops refused to cooperate with Augustine, the archbishop warned them that they would suffer at the hands of the Saxons, a prophecy connected to the later massacre of the monks of Bangor-Is-Coed by King Æthelfrith of Northumbria (HE II.2). The story serves Bede's narrative purpose of vindicating the Canterbury mission against Celtic resistance, and the prophetic framework should be read with that in mind.