Which event in Prague is considered the trigger of the Thirty Years' War?

The event in Prague considered the trigger of the Thirty Years' War was the Defenestration of Prague.

More precisely, historians usually mean the Third Defenestration of Prague on 23 May 1618. Protestant nobles in Bohemia confronted two Catholic representatives of the Habsburg king, Jaroslav Bořita of Martinice and Vilem Slavata, along with their secretary, Filip Fabricius. The nobles threw all three from a window of Prague Castle.

The victims survived the fall, reportedly landing in a pile of refuse or soft ground, but the political consequences were far more serious. The Protestant estates formed a rebel government, rejected Habsburg authority, and later chose Frederick V of the Palatinate as king of Bohemia. The Habsburg response turned the revolt into a wider war.

Prague had experienced other defenestrations, including one in 1419 that helped ignite the Hussite Wars. That is why the wording can seem imprecise. In the context of the Thirty Years' War, however, “Defenestration of Prague” conventionally identifies the 1618 incident.