Schwarzer Freitag is a stock trading game with a built-in bubble to explode. The players are buying shares on a growing stock market, but at one time in the game the prices will crash. The players try to find out the right time to sell their shares and buy gold and silver (these are the victory points), before the market crashes.
The actions of all players change the probability of the crash. When they start selling, the probability of the crash will get higher. If they only buy shares then the crash will come later.
To win the game, you have to have a close look at what the others do to be in the position to sell the highest share at the right time, to get the most money. But the silver/gold price will rise too, and you can also win by buying silver/gold early in the game.