Where Does the Word 'Mayday' Come From? The amazing origin of a famous distress call
Merriam Webster

[snip]

Mayday is an internationally recognized radio word to signal distress. It's used mostly by aircraft and boats, and most of us are happily only familiar with it through TV and fiction. It appears as both an interjection ("Mayday! Mayday!") and to modify a noun ("a mayday signal"). The May Day that refers to the first of May has been in English for a very long time—back to the 1200s, in fact—but it’s not what inspired the call for help.

Mayday first came into English in 1923. There was a lot of air traffic between England and France in those days, and evidently there were enough international problems over the English Channel that both parties wanted to find a good distress signal that everyone would understand. But surely there already was a distress signal that everyone understood? There was—S.O.S.—but there were some problems with it:

Owing to the difficulty of distinguishing the letter "S" by telephone, the international distress signal "S.O.S." will give place to the words "May-day", the phonetic equivalent of "M'aidez", the French for "Help me."

—"New Air Distress Signal," The Times [London], 2 Feb. 1923