Wordorigins.org

This distress call is a phonetic representation of the French m’aider, literally help me. In this case the change from the French is deliberate and not a result of folk etymology. Its use dates to the 1927 International Radio Telegraph Convention:

Rules apply to the radio telephone distress call which consists of the spoken expression Mayday, (corresponding to the French pronunciation of the expression “m’aider").

(Source: Oxford English Dictionary, New Edition)