This is the emblematic event of the French Wars of Religion. On August 24, after the marriage of Henry of Navarre (the future Henry IV) and Marguerite de Valois (daughter of Catherine de Medici and sister of the king), most of the Protestant leaders present in Paris at time were assassinated by the Duke de Guise’s party. The situation degenerated into a widespread massacre, even extending outside the capital.

Article : St. Bartholomew’s Day (24th August 1572)