where did the use of the term mayday as a distress signal come from?
there’s may day. first day of may. dance around a pole. and it’s apparently like our (canada’s) labour day in some european countries.
but how did it become a term for distress? or, if it has nothing at all to do with may day, where’d it come from?
This seems too simple, but I believe it comes from m’aidez meaning “Help me” in French (Once the “de rigueur” language for international communication).
The “M’aidez” explanation is the one I’ve heard, but the French don’t use that to call for help; the phrase is “au secours.” All I can guess is that an English-speaker went to a French dictionary to try to put the phrase “help me” into French.
I’ve read the French explanation too. Incidentally, it’s repeated three times: “Mayday! Mayday! Mayday!” This is because anything in threes is considered a distress signal. “SOS” is **… — …*, a triangle of fires indicates “I need help down here!”, flying in a triangle indicates a radio failure (my helicopter instructor would test me on this. “What would you do if you had a radio failure right now?” I told him, “I’d land in that parking lot down there and find a pay phone!”), let go with three shots and it means you need help ("But we’re almost out of arrows!)…
If it’s not an emergency, but an urgency, you’re supposed to say “Pan-pan”; but I’ve never heard anyone say that.
May•day "ma-'da\ [F m’aider help me] (1927)
— an international radio-telephone signal word used as a distress call
(C)1996 Zane Publishing, Inc. and Merriam-Webster, Incorporated. All rights reserved.
Is french, see 
IIRC the three levels of emergency are:
Emergency pronounced EmergenSay, so that it is easily distinguished from people talking about emergencies.
Panic pronounced PANic.
and Mayday.
These are repeated three times in order to be distinguished from the casual usage of the words. If it is a true emergency you are allowed to broadcast the call on frequencies that you otherwise aren’t allowed to use.
Keith
VE5KIS
I hadn’t heard that, Keith–gotta cite?
As for the fact that the French don’t actually say “m’aider/m’aidez” themselves, so what? The Germans don’t say “gesundheit.” No worries. Etymologies don’t always have to make sense. 
Johnny L.A. said: “If it’s not an emergency, but an urgency, you’re supposed to say “Pan-pan”; but I’ve never heard anyone say that.”
Actually, there are three levels of emergency signals used in marine radio communication. From the FCC’s page on SHIP RADIO STATIONS:
I have heard all of them used over the radio. Mayday is obviously for distress calls.
I have usually heard the Coast Guard make Pan-Pan calls to warn of a vessel that is overdue.
Securite calls are fairly common. I usually hear them in reference to navigational hazards.
Bill
I don’t have a citation from a print source, but this question was addressed on a web site that I enjoy called The Word Detective. http://www.word-detective.com/back-n.html#mayhem
He’s got lots of interesting word and phrase origins there for your amusement. (If, like me, you are geeky enough find etymologies amusing that is.)
Ignore my post above. I was posting under the influence of ignorance. 
Keith