Common custom in many of the northern countries wanted the just graduated high-school students to celebrate the last day together before leaving for the military service.
Nowadays that the conscription is not compulsory any more, the parade remains an occasion to vent and celebrating getting drunk by the streets of the city. Every year, for a whole day, Stockholm's " Gymnasium" students wander uninterruptedly by the streets on luxurious cars, limos, or even on trailers and trucks playing loud music invading the urban spaces usually half-deserted.
Beer and champagne mark the end of the formative path, in a "raptus" of impulsive break of the rigid Scandinavian schemes.