From Wikipedia:
International Workers’ Day, also known as Labour Day in some countries and often referred to as May Day, is a celebration of labourers and the working classes that is promoted by the international labour movement and occurs every year on 1 May, or the first Monday in May.
[…]
The 1 May 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 four days later.
Gemma is wrong. Or at least I have never heard it as one combined word. As the examples themselves show, it is normally used as adjective+noun. And while you can combine it, I have never seen that till now.
And to OPs question: I don’t know any such word in the German language. It might be a regional thing or it might just be something Vonnegut invented himself.