Beltane was one of four Gaelic seasonal festivals: Samhain (~1 November), Imbolc (~1 February), Beltane (~1 May), and Lughnasadh (~1 August). Beltane marked the beginning of the pastoral summer season, when livestock were driven out to the summer pastures. Rituals were held at that time to protect them from harm, both natural and supernatural, and this mainly involved the "symbolic use of fire". There were also rituals to protect crops, dairy products and people, and to encourage growth. The aos sí (often referred to as spirits or fairies) were thought to be especially active at Beltane (as at Samhain) and the goal of many Beltane rituals was to appease them. Most scholars see the aos sí as remnants of the pagan gods and nature spirits. Beltane was a "spring time festival of optimism" during which "fertility ritual again was important, perhaps connecting with the waxing power of the sun". Before the modern era.
Beltane (the beginning of summer) and Samhain (the beginning of winter) are thought to have been the most important of the four Gaelic festivals. Sir James George Frazer wrote in The Golden Bough: A Study in Magic and Religion that the times of Beltane and Samhain are of little importance to European crop-growers, but of great importance to herdsmen. Thus, he suggests that halving the year at 1 May and 1 November dates from a time when the Celts were mainly a pastoral people, dependent on their herds.
The earliest mention of Beltane is in Old Irish literature from Gaelic Ireland. According to the early medieval texts Sanas Cormaic and Tochmarc Emire, Beltane was held on 1 May and marked the beginning of summer. The texts say that, to protect cattle from disease, the druids would make two fires "with great incantations" and drive the cattle between them.
According to 17th-century historian Geoffrey Keating, there was a great gathering at the hill of Uisneach each Beltane in medieval Ireland, where a sacrifice was made to a god named Beil. Keating wrote that two bonfires would be lit in every district of Ireland, and cattle would be driven between them to protect them from disease. There is no reference to such a gathering in the annals, but the medieval Dindsenchas includes a tale of a hero lighting a holy fire on Uisneach that blazed for seven years. Ronald Hutton writes that this may "preserve a tradition of Beltane ceremonies there", but adds "Keating or his source may simply have conflated this legend with the information in Sanas Chormaic to produce a piece of pseudo-history." Nevertheless, excavations at Uisneach in the 20th century found evidence of large fires and charred bones, showing it to have been ritually significant.
Beltane is also mentioned in medieval Scottish literature. An early reference is found in the poem 'Peblis to the Play', contained in the Maitland Manuscripts of 15th- and 16th-century Scots poetry, which describes the celebration in the town of Peebles.