There are at least three well known definitions that are associated with May 1st or May Day, two are celebrations and one is an international distress call.

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A May Pole Celebration and Dance in Modern Bavaria

Let's start with the oldest the Maypole Dance on May Day or May 1st.  This is believed to have started in perhaps the 16th century by Germanic tribes as a pagan celebration of fertility rituals.  Though they've never started a shooting war over it, England and Sweden also co-claim with Germany the commencement of the May Pole dance.  The first dances were conducted around trees and later a tall pole was substituted.  As the dances go around the pole, modern ribbons in hand tied to the top of the pole they get closer and closer to each other (mating ritual) and when they're as close to the pole as possible they start dancing in the opposite direction to unwind.

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A Few Months of Hard Labor in The Gulag Will Teach this Youngster Not to Stick His Head in Between Marx and Lenin!

The next oldest celebration of May Day deals with the labor movement, which in the 20th century morphed into a major communist celebration in countries with that type of government. But it actually started in the US.  On May 1st, 1886 American Labor Unions went on strike with the demand that workers should not be allowed to work more than 8 hours a day.  This wasn't an entirely peaceful protest, a few days later several people were killed when a bomb was set off in Chicago's Haymarket Square on May 4th.  While no immediate effect was felt, workers eventually welcomed an 8 hour day as 'normal', and for years May 1st included labor celebrations across the country - that is until the rising communists countries co-opted the day as a 'Workers Unite' day and began having big celebrations and marches.

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When You Send out a "MAYDAY" and Only a Small Sailboat Comes to Help

Finally - if you've ever seen a movie or television show in which a ship or plane is in imminent danger of crashing or sinking you've no doubt heard the radio-telegraph operator on the vessel send out "MAYDAY" requests for help and as an indication they are in extreme, life threatening distress.

In 1923 a British Sr. Radio Officer Frederick Stanley Mockford, devised a special word (Morse code and the S.O.S. was already giving way to voice communications on wireless radio) that could easily be understood by almost anyone.  "Help" was no good as it could be used in normal conversation, "let me finish darning my socks and I'll help you with your buttons", and could be confusing to non-English speaking people.  So Mockford took the French word for "help me", which is 'm'aider' and anglicized the spelling of the French pronunciation into 'May-Day".

I could find no evidence that teachers scream 'May-Day' on this day to forewarn parents their little angels are about to be home-bound for 9 weeks.

So whether you're out marching for workers, dancing around a festive May Pole or (hopefully not) on a sinking boat in the Gulf of Mexico, enjoy today. May Day.