Originally a day of carnivalesque suspension of the social order of work and control, prevalent among peasant communities in mediaeval Europe. A continuation of earlier pagan festivities which continued in a syncretic way during the Christian era, with rituals such as the maypole retained. (This was originally a pagan fertility ritual).
It became a focus of international resistance for two reasons: firstly, because the campaign for Mayday to be made into a holiday was an important early campaign of the workers' movement, and secondly and more importantly, because of the Haymarket martyrs, anarchists stitched up over a bombing at a protest.
It derives from the French "m'aidez", pronounced "mayDAY," and means literally "help me."
SOS is the morse code version
MAYDAY is reconized internationally as an emergency code on any type of radio system, and should be taken seriously.
The story of May Day:
In early May of 1886, anarchist and labor activists rioted in Haymarket Square in Chicago in the name of winning an eight-hour workday. During that time, most capitalist bosses required their workers to work for 14 hours a day with little breaks or days off. During the riot, someone threw a bomb. Five anarchists were accused of bomb-throwing or a conspiracy to do such and were tried, later convicted, and hanged. May Day became a day to remember their sacrifice as well as the sacrifices of all laborers who must work under the conditions of wage slavery.
Legend goes that the first May Day was held by a professor in the 60's, along with several of his students.
("Who said Mayday was a sellout?" by David Momphard, Taipei Times, Friday, Jan 07, 2005)
It is used to signal a life-threatening emergency primarily by mariners and aviators but in some countries local organisations such as police forces, firefighters, and transportation organizations may also use the term. The call is always given three times in a row ("Mayday Mayday Mayday") to prevent mistaking it for some similar-sounding phrase under noisy conditions, and to distinguish an actual Mayday call from a message about a Mayday call.
A mayday situation is one in which a vessel, aircraft, vehicle, or person is in grave and imminent danger and requires immediate assistance. Examples of "grave and imminent danger" in which a mayday call would be appropriate include fire, explosion or sinking.
Civilian aircraft in the UK and Europe are encouraged to use the following format:
MAYDAY, MAYDAY, MAYDAY; Name of station addressed; Aircraft callsign; Nature of emergency; Intentions of the pilot; Present position (including Altitude or Flight level); Pilot's qualifications; Any other useful information (number of souls on board.)