Mayday!

Let’s see what a quick scan of the usual sources turns up:

As a day of celebration May Day has ancient origins, and it can relate to many customs that have survived into modern times. Many of these customs are due to May Day being a cross-quarter day, meaning that it falls approximately halfway between an equinox and a solstice.

The earliest May Day celebrations appeared in pre-Christian Europe, as in the Celtic celebration of Beltane, and the Walpurgis Night of the Germanic countries. Many pre-Christian indigenous celebrations were eventually banned or Christianized during the process of Christianization in Europe. As a result, a more secular version of the holiday continued to be observed in the schools and churches of Europe well into the 20th century. In this form, May Day may be best known for its tradition of dancing the Maypole and crowning of the Queen of the May. Today various Neo-pagan groups celebrate reconstructed (to varying degrees) versions of these customs on May 1st.

The day was a traditional summer holiday in many pre-Christian European pagan cultures. While February 1st was the first day of Spring, May 1sst was the first day of summer; hence, the summer solstice on June 25 (now June 21) was Midsummer. In the Roman Catholic tradition, May is observed as Mary's month, and in these circles May Day is usually a celebration of the Blessed Virgin Mary. In this connection, in works of art, school skits, and so forth, Mary's head will often be adorned with flowers, often roses. Fading in popularity since the late 20th century is the giving of "May baskets," small baskets of sweets and/or flowers, usually left anonymously on neighbors' doorsteps.

Hope that helps clarify things a bit!