Originally, back in the day, the first university students would wear their graduation hats and take a horse carriage ride around town. The tradition has disintegrated somewhat since then. These are college students who are celebrating 2 years of paid study.
The banners read;
"Two years of slime*, now it's party time"
"A real hat is blue, if it's red you schmooser**, you took 3 years you looser"
* a liberal translation of Mackerel, made to rhyme.
** a creative translation for Achiever.