It's kind of a catch all that originates in pretty much all of the pre-Christian European religions - Walpurgis Night, Beltane, etc. Basically a start of spring festival. Dance around a May pole, leave people baskets of flowers, sing and drink a lot.
In the United States, it is celebrating the coming of Spring.
Many countries celebrate the workers, it is a labor day celebration.
When I was growing up we did a May pole with ribbons and danced around it with flowers on our clothes and in our hair.