(Okay, I know it's the fourth of May, but I tend to run about three days behind the rest of the world.)
First of May is one of my favorite circus/carnival terms, perhaps because it sounds so pleasant when, in fact, it's more of an affectionate insult. I guess it's like newbie/noob/n00b or green, as it refers to someone in their first season with the circus.
I can't find much about the term other than its definition, so I'm not sure of the origins or applications. I have an impression that it is used for workers and performers alike, though I'm basing that only on Feiler's Under the Big Top, in which he explains that he, working as a clown (which seems to fall somewhere in the hierarchy between worker and performer), was called a First of May. I was surprised to find no listing for it in James Taylor's "Carny Lingo," which leads me to believe this is more of a circus term than a carnival term. (I'm guessing, but Taylor's list is otherwise so thorough that I am relatively comfortable with this guess.)