Mayday!

Yes, of course it's a big holiday, known as Labo(u)r Day in about 30 countries on my list.
"For most countries, "Labour Day" is synonymous with, or linked with, International Workers' Day, which occurs on 1 May."
See this article for a number of countries which celebrate "Labo(u)r Day" at completely different times of year.
https://en.wikipedia.org/wiki/Labour_Day
This observance is of late 19th century origin:
"The International Workers Congress held in Paris in 1889 established the Second International for labor, socialist, and Marxist parties. It adopted a resolution for a "great international demonstration" in support of working-class demands for the eight-hour day. The date was chosen by the American Federation of Labor to commemorate a general strike in the United States, which had begun on 1 May 1886 and culminated in the Haymarket affair on 4 May."
https://en.wikipedia.org/wiki/International_Workers%27_Day
But May 1 is "Spring Day" in Estonia, and "Vappu" in Finland. And this brings us to a much older holiday, in honour of the beginning of summer or of St.Walpurga (who was English!).
https://en.wikipedia.org/wiki/May_Day
https://en.wikipedia.org/wiki/Walpurgis_Night
Ob/sci.lang
"Mayday!" is an international radio distress signal.
It comes, we are told, from French "m'aidez!" (help me).
But we were taught in school that, while the object pronoun (me) is normally proclitic to the verb, in the imperative it must follow it (aidez-moi!). The use as a distress call dates back only to the 1920s, so I don't think we can appeal to some earlier stage of the language to justify "m'aidez". The other possibility is that it's really "m'aider", which (OED suggests) could be either short for "venez m'aider" (come and help me!), or perhaps the "imperative infinitive". This expression gave me pause. I think I have encountered French infinitives used with imperative force, but my experience doesn't tell me what contexts they are used in and with what pragmatic force. Of course you would get the infinitive with "veuillez..." or "voulez-vous..." (kindly, would you please), but those do not have the urgency appropriate to a distress call.