Triggerfish is a holiday celebrated annually in the United Empire of Earth (UEE) on 1 April. Those who observe it play pranks and jokes on one another or participate in hoaxes, shouting "Triggerfish!" once a jape is complete.
Though the holiday has origins stretching as far back as the 16th century SEY, it first began to be called Triggerfish in the 2600s, when it became tradition for residents of Borea (Magnus II) to try and convince visitors that a secret fourth planet named "Triggerfish" existed in the Magnus system. Some outlaws would give victims coordinates to the planet, only to ambush them once they arrived. To put an end to this dangerous practice, the UEE launched an information campaign refuting the claim that Triggerfish existed. This spread the name outside its system of origin. Triggerfish soon became a nickname for a gullible person, and the name became the most common way to refer to the holiday.