April Fool's Day is Waluigi's favorite time of the year. It's a semi-holiday set on the very first day of April, and its premise is to prank as many idiots as you can. People that have no sense of humor often find this their least favorite day of the year.
Believe it or not, but April Fool's Day isn't called so in every other country. In fact, it has lots of names, each with a completely different origin, all based on pranking people on April 1st. Crazy huh?
The English name obviously comes from "fool", which originally was synonymous for "jester", whose job was to make practical jokes to amuse the royal court of medieval times. However, the word "fool" nowadays stands synonymous for "idiot" or "dumbass". You could say that April Fool's day is a double pun on the prankster and the target.
The Dutch name for this day is simply "één april", which is just April 1st. BOOOOOOORING.
The French call this day "Poisson d'Avril", which means "Fish of April". This apparently comes from the fact that in the olden days, pranksters attach paper fish on their target's back without being noticed. However, SJWs might take offense to that, because they think that calling a pranks day after a fish equals all fish being an idiot, and thus would offend all fish-gendered people and other fish-kin, including actual fish. But we can all agree that attaching a paper fish to someone's back makes little to no sense, as there's no punchline; attaching a "Kick Me" sign gives a much funnier result. The italian name has the same origins, btw.
The German name is "Aprilscherz", which means April Joke. Laaame.