Aldersgate Day, or Wesley Day, is an anniversary observed by Methodist Christians on 24 May. It recalls the day in 1738 when Church of England priest John Wesley attended a group meeting in Aldersgate, London, where he received an experience of assurance of his New Birth. This was the pivotal event in Wesley's life that ultimately led to the development of the Methodist movement in Britain and America.In the calendars of the Methodist Church of Great Britain and the United Methodist Church the event is publicly commemorated in church services on the Sunday before 24 May (or on the anniversary itself if that date falls on a Sunday), called Aldersgate Sunday.

John Wesley (; 28 June [O.S. 17 June] 1703 – 2 March 1791) was an English cleric, theologian, and evangelist, who was a leader of a revival movement within the Church of England known as Methodism. The societies he founded became the dominant form of the independent Methodist movement that continues to this day.

Educated at Charterhouse and Christ Church, Oxford, Wesley was elected a fellow of Lincoln College, Oxford, in 1726 and ordained as an Anglican priest two years later. At Oxford, he led the "Holy Club", a society formed for the purpose of the study and the pursuit of a devout Christian life; it had been founded by his brother Charles and counted George Whitefield among its members. After an unsuccessful ministry of two years, serving at Christ Church, in the Georgia colony of Savannah, he returned to London and joined a religious society led by Moravian Christians. On 24 May 1738, he experienced what has come to be called his evangelical conversion, when he felt his "heart strangely warmed". He subsequently left the Moravians and began his own ministry.

A key step in the development of Wesley's ministry was, like Whitefield, to travel and preach outdoors. In contrast to Whitefield's Calvinism, Wesley embraced Arminian doctrines. Moving across Great Britain and Ireland, he helped form and organise small Christian groups (societies) that developed intensive and personal accountability, discipleship, and religious instruction. He appointed itinerant, unordained evangelists – both women and men – to care for these groups of people. Under Wesley's direction, Methodists became leaders in many social issues of the day, including the abolition of slavery and prison reform.

Although he was not a systematic theologian, Wesley argued for the notion of Christian perfection and against Calvinism—and, in particular, against its doctrine of predestination. His evangelicalism, firmly grounded in sacramental theology, maintained that means of grace sometimes had a role in sanctification of the believer; however, he taught that it was by faith a believer was transformed into the likeness of Christ. He held that, in this life, Christians could achieve a state where the love of God "reigned supreme in their hearts", giving them not only outward but inward holiness. Wesley's teachings, collectively known as Wesleyan theology, continue to inform the doctrine of Methodist churches.

Throughout his life, Wesley remained within the established Church of England, insisting that the Methodist movement lay well within its tradition. In his early ministry years, Wesley was barred from preaching in many parish churches and the Methodists were persecuted; he later became widely respected, and by the end of his life, was described as "the best-loved man in England".