May opens with Beltane, the Gaelic fire festival that marked the turn to summer, when cattle were driven between paired bonfires for protection before the move to summer pasture. Modern Pagans keep it as one of the four great cross quarter stations of the Wheel of the Year, and its eve survives across northern Europe as Walpurgis Night, a bonfire night that German folklore filled with witches on the Brocken.
The full moon of the month is Vesak for most of the Buddhist world, the single day on which Theravada tradition places the birth, the awakening and the final passing of the Buddha. Homes and temples are lit with lanterns, the infant Buddha image is bathed with sweet tea or scented water, and since 1999 the United Nations has formally recognised the day, which falls in May in most years and occasionally in early June. The same lunar reckoning usually brings Lag BaOmer, the bonfire break in the Jewish counting of the Omer, into May as well.
For Catholics the whole month belongs to Mary. May devotions fill parish churches, statues are crowned with flowers in the rite popularly called the May crowning, and the month holds Our Lady of Fatima on the thirteenth, the anniversary of the first apparition reported by three shepherd children at Fatima in Portugal in 1917. The Baha'i calendar adds one of its holiest days near the month's end: the Declaration of the Bab on the twenty third of May, marking the evening in Shiraz in 1844 when the Bab announced his mission, the moment from which the Baha'i era is counted.
Sources Celtic Culture: A Historical EncyclopediaJohn T. Koch (ed.) with The Buddhist World of Southeast AsiaDonald K. Swearer, The Oxford Dictionary of the Christian Church, 3rd revised editionF. L. Cross and E. A. Livingstone (eds.)