For those who do recognize May Day as a holiday, it can be for two separate reasons.
Rites of the season
May Day pre-dates Christianity and in its early days marked the halfway point between the spring equinox and summer solstice (in the Northern Hemisphere). It celebrated fertility and the day was spent dancing, feasting, and making merry. In ancient Rome, May Day was named for the Roman goddess of flowers, Floralia.
May Day's roots are astronomical. It’s one of the year’s four cross-quarter days, or a 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 Feb. 2, Lammas on Aug. 1 and Halloween on Oct. 31.
In the late 19th century, May Day took on a second meaning. Laborers fighting for workers' rights, including an eight-hour workday, began what became an infamous protest — the Haymarket Affair in Chicago — on May 1, 1886. A few years later, a group called the International Socialist Conference declared it "International Workers' Day."
That pole with all the ribbons
A secular event by the Middle Ages, the best known way to celebrate May Day — to this day in some locations — was to dance around the Maypole, a large pole with dozens of ribbons fastened to the top. Dancers, each with a ribbon in hand, dart around the pole in specific steps to weave the ribbons down the pole. In Great Britain, nearly every village cut its own Maypole for May Day.
Mayday! Mayday! Mayday!
"Mayday," an international radio distress signal used especially by ships and aircraft, has no connection to May 1 or either reason some folks celebrate May Day. It is thought to be an anglicization of the French "m’aidez" or "m’aider," meaning "help me."
Resources: Farmer's Almanc, Oxforddictionaries.com, earthsky.org, Mother Nature Network.