From Max Cryer‘s  book ‘Is It True?’

The distress call ‘mayday’ is English for the French term m’aidez.

 Using the word ‘mayday’ dates from 1923, when a senior radio officer at Croydon Airport was asked to think of an easily understood  word which could indicate distress.

At the time there was considerable  air traffic between Croydon and Paris, so radio officer Frederick Mockford  invented the term  ‘mayday’, supposedly based on the French  ‘venez m‘aidez’ (meaning ‘come help me’) which he assessed that even in its abbreviated form would be understood by both French and British staff.   The term is widely  understood – and  when necessary used – in English, and in 1927  took some equality  with the SOS distress signal (1908).

But there is a  problem, because saying ‘m’aidez’ would never occur at all in genuine French – it simply isn’t correct. French people tend to call ‘ Aidez-moi’ ( a polite request for help) or ‘au secours’ (‘to the rescue’).  However the mangled  ‘mayday’  has become established among English speakers,  and in some circumstances French people will  call SOS or sometimes ‘mayday’ but using it as  an ‘English’ expression.