“Mayday” (m’aider is French meaning ‘help me’) is a life-threatening emergency voice signal primarily used by aviators and mariners, but also used by law enforcement and firefighters. It is always given three times in a row to ensure the message is clear. It was selected by Frederick Stanley Mockford, a London based senior airport radio officer, in 1923. In 1927, it officially replaced the Morse code distress call, SOS.
It is unrelated to the May Day holiday.