Recently created custom of Australian football
If a team loses a major game, they gather at a pub or club and get extremely drunk on the following Monday. They let their hair down, drink a lot of beer, get wasted and sometimes make a public spectacle of themselves.
The justification seems to be that, they deny themselves the pleasure of drink to maintain fitness and peak performance, and so they miss out on raging with their friends on Friday and Saturday nights. If they win the game, that is their reward for the self denial, so they can continue the training and playing and aiming to win the competition. If they lose and are knocked out of the competition, they commiserate and blow off steam in a Mad Monday orgy of booze and self degradation. Legends are born, rumours spread.
(I only found out about this recently when I turned up to my favourite hotel on a Monday to find it closed for a private function, and full of drunken footballers pretending to be stage diving off the tiny 2 foot stage. It was the Sydney Swans, an AFL team, who had lost a big game the previous Friday and been knocked out of the Grand Final.)