How smug do you think Fred and George would have been when they learned about the muggle holiday of April Fool’s falling on their birthday?
April Fool’s day actually dates back to the transition from the Julian to the Gregorian calendar. Wizards almost certainly know of it.
Alternative idea. Wizards originated April Fools as a day that they could preform magic in front of muggles under the guise of it being a trick.
Wizard: *transforms a person into a frog*
Muggle: A sorcerer!
Wizard: *about to invent April Fools* Oh haven’t you heard…