Copied from article Earth Sky (one of my favorites websites)
It’s May Day 2013 – May 1 on the calendar. May Day is an ancient spring festival in the Northern Hemisphere. It’s an astronomical holiday, one of the year’s four cross-quarter days, or day that falls more or less midway between an equinox and solstice – in this case the March equinox and June solstice. The other cross-quarter days are Groundhog Day on February 2, Lammas on August 1 and Halloween on October 31. May Day also stems from the Celtic festival of Beltane, which was related to the waxing power of the sun as we move closer to summer. At Beltane, people lit fires through which livestock were driven and around which people danced, moving in the same direction that the sun crosses the sky.
May Day on May 1 is one of four cross-quarter days, midway between an equinox and a solstice. It stems from the ancient festival of Beltane, which relates to the waxing power of the sun at this time of year. Its most recognized tradition is the Maypole dance, in which people wrap ribbons around a tall pole.