The “mayday” procedure word was conceived as a distress call in the early 1920s by Frederick Stanley Mockford, officer-in-charge of radio at Croydon Airport, London. He had been asked to think of a word that would indicate distress and would easily be understood by all pilots and ground staff in an emergency.

Since much of the air traffic at the time was between Croydon and Le Bourget Airport in Paris, he proposed the term “mayday”, the phonetic equivalent of the French m’aidez (“help me”) or m’aider (a short form of venez m’aider, “come [and] help me”).

The term is unrelated to the holiday May Day. Following tests, the new procedure word was introduced for cross-Channel flights in February 1923.

The previous distress call had been the Morse code signal SOS, but this was not considered suitable for voice communication, “[o]wing to the difficulty of distinguishing the letter ‘S’ by telephone”.

In 1927, the International Radiotelegraph Convention of Washington adopted the voice call “mayday” as the radiotelephone distress call in addition to the SOS radiotelegraph (Morse code) signal.