Why is a "ball" called a "ball"? For that matter, why is a "ples" called a "ples"? A ball meaning an organized event for a large number of people involving entertainment, food, music and dancing - that has nothing to do with the globular objects of different sizes used in various sports.

That word comes from Old German or Old Norse, but the 7th Annual Ball form of the word comes from Old French "bal", meaning a dance, and "baller", meaning to dance. It's connected of course with the word "ballet" (say /balé/), meaning a little dance, and what is more, it's the root of the word "ballad", meaning the kind of song which people can dance to, being relatively slow and repetitive, and sentimental or romantic, so people can sing along with it while they are dancing - the old equivalent of "lepák" then.