May Day's roots lie in agriculture, as with many early holidays. Spring celebrations celebrated the emergence of sown fields filled with song and dance. Yellow May flowers were decorated on doors of houses and on livestock as well as special bonfires lit for the occasion. It was a festival of Beltane celebrated by the Gaelic people during the Middle Ages. People lit a large bonfire and danced at night to celebrate Beltane, which means "Day of Fire."
Source: Almanac
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