A 19th century painting by Édouard Debat-Ponsan, depicting Catherine de' Medici (in black) viewing the carnage of the St. Bartholomew's Day massacre (on this day in 1572).
The St. Bartholomew's Day massacre (Massacre de la Saint-Barthélemy in French) in 1572 was a targeted group of assassinations, followed by a wave of Roman Catholic mob violence, both directed against the Huguenots (French Calvinist Protestants), during the French Wars of Religion. Traditionally believed to have been instigated by Catherine de' Medici, the mother of King Charles IX, the massacre took place six days after the wedding of the king's sister Margaret to the Protestant Henry III of Navarre (the future Henry IV of France). This marriage was an occasion for which many of the most wealthy and prominent Huguenots had gathered in largely Catholic Paris.Much much more at Wikipedia and elsewhere.
The massacre began on 23 August 1572 (the eve of the feast of Bartholomew the Apostle), two days after the attempted assassination of Admiral Gaspard de Coligny, the military and political leader of the Huguenots. The king ordered the killing of a group of Huguenot leaders, including Coligny, and the slaughter spread throughout Paris. Lasting several weeks, the massacre expanded outward to other urban centres and the countryside. Modern estimates for the number of dead vary widely, from 5,000 to 30,000.
The massacre also marked a turning point in the French Wars of Religion. The Huguenot political movement was crippled by the loss of many of its prominent aristocratic leaders, as well as many re-conversions by the rank and file, and those who remained were increasingly radicalized. Though by no means unique, it "was the worst of the century's religious massacres." Throughout Europe, it "printed on Protestant minds the indelible conviction that Catholicism was a bloody and treacherous religion".