‘AS REIMAGINED ANCIENT TRADITIONS GO, Beltane is one of the flashier ones. Modern events such as Edinburgh’s iconic Beltane Fire Festival and smaller bonfire gatherings, particularly in Ireland and Scotland, light it up around April 30 and May 1 each year in what many consider a symbolic cleansing and celebration of renewal. Even before the bonfires get blazing, modern Beltane—often rolled in with more general May Day festivities—is a visual feast featuring colorful flowers, dancing, May Queens, Green Men, and other revelry.
The ancient roots of Beltane are more mundane: It had a lot to do with cows, and it wasn’t on May 1. It did, however, mark the most important transition of the year.
…Along with the solstices and equinoxes, the four quarter days, also known as cross-quarter days, are the most prominent dates in the Celtic calendar. Each quarter day occurs halfway between a solstice and an equinox. The cross-quarter day most familiar to us comes between the fall equinox and winter solstice: Samhuinn, also known as Samhain, or, of course, Halloween, which heralded the arrival of the dark, lean season of winter. The cross-quarter day of Imbolc, halfway between the winter solstice and the vernal equinox, marked the start of lambing season. It was also a time when early agrarians were anxiously waiting for signs of spring, and is a precursor to the modern Groundhog Day.
There’s also Lughnasadh, or Lúnasa, the cross-quarter day marking the beginning of harvest time, halfway between the summer solstice and autumn equinox. (Alas, Lughnasadh has not inspired any traditions of prophetic rodents or dressing up to demand treats and threaten tricks, and remains relatively obscure today.)
For the early inhabitants of the British Isles, the most important cross-quarter day of the ancient agrarian calendar wasn’t Imbolc, Lughnasadh, or Samhuinn. It was Beltane (other spellings include Beltaine and Bealltainn), which marked the start of summer. It was celebrated midway between the spring equinox and summer solstice which, astronomically speaking, varies each year but falls around May 5 or 6….’
— via Atlas Obscura