According to legend, a meeting of lawmakers was supposed to occur in Augsburg on April 1, 1530 in order to unify the state coinage. Unscrupulous speculators, who had knowledge of it beforehand, began to trade currencies in preparation, to profit from the change. However, because of time considerations, the meeting didn't take place, and the law wasn't enacted. So the speculators who had bet on the meeting occurring, lost their money and were ridiculed. German folklore has it that this was the origin of the custom of playing pranks on April 1.
Augsburg, circa 1500