John Collier's Queen Quinevere's maying (1900)
Tonight at sundown begins Beltane (also spelled Beltain, Bealtaine, Bealltainn, Boaltinn, and Boaldyn), a Gaelic holiday marking the beginning of summer (which in the Gaelic calendar takes place at the midpoint between the spring equinox and summer solstice). Historically it was observed throughout the British Isles.
Beltane is mentioned in some of the earliest surving medieval Irish literature (eg. Sanas Cormaic and Tochmarc Emire) . It was one of the four Gaelic seasonal festivals along with Imbolc, Samhain, and Lugnasadh. but was, not surprisingly, attacked by intolerant Christians. By the mid-20th century, Beltane observances had almost died out although there have been efforts to revive it.
Betlane observers (image source: Twilight Language)
On Beltane rituals were undertaken to ensure a good year in the fields. This was done with the burning of bonfires through which farm animals would be driven. According to 17th century historian Geoffrey Keating, animal sacrifices were made to the god, Beil, on the the hill of Uisneach in Ireland. Additionally, farmers led processions around their fields whilst carrying grains, farm implements, Beltane well water, and vervain or rowan. Rituals were performed along the way at each of the four cardinal points of the compass.