Mine is that International Workers Day is May 1st to commemorate the Haymarket Affair, which happened in America where labor day is conspicuously much later in the year.
Don't know if this qualifies as trivia, but I love that Marx wrote "The Poverty of Philosophy" as a direct "fuck you" to Proudhon's "The Philosophy of Poverty". It cracks me up every time I think about it.
Lol he was such a snarky little shit too, I love reading critique of gotha and seeing him get continually more pissed and tearing apart even single words as being opportunist or vague.