Oh okay: shortly, it's a character in the series that always, and I mean always, wears a suit. He even sleeps in a silk suit pyjamas! Anyways he always tries to get the other characters to "suit up!". He sometimes do an other type of suit up, like "snow suit up", "slut suit up", "law suit up"
So on Suit-Up day you are supposed to wear a suit! I don't have any, but some guys in my class did it, and I know e.g. my little brother does it too. A fun way to spice up the everyday life with, and you most agree with me that guys look better in a suit ;)
Oscar in my class and I (doesn't he look a bit like Barney?)
I finish with the Suit Up Song/ Nothing suits me like a suit: