Short answers, pulled from the story.
The French Wars of Religion were a series of civil wars between French Catholics and Protestants, called Huguenots, from 1562 to 1598. Between two and four million people died from violence, famine, or disease caused by the conflict, and it severely damaged the power of the French monarchy.
The agreed beginning of the French Wars of Religion is the Massacre of Vassy in 1562. The fighting ended in 1598 with the Edict of Nantes and the Peace of Vervins, signed on the 2nd of May 1598, though the 1620s Huguenot rebellions lead some historians to date the conclusion to the Peace of Alès in 1629.
The St. Bartholomew's Day massacre began on the morning of the 24th of August 1572, when kill squads struck at the Huguenot leadership in Paris and Admiral Coligny was killed around 4am. The violence spread for five days and across more than a dozen cities, and historians estimate that 2,000 Huguenots were killed in Paris and perhaps 10,000 in all of France.
Henry of Navarre, proclaimed Henry IV, ended the French Wars of Religion by converting to Catholicism on the 25th of July 1593, reputedly saying Paris vaut bien une messe, Paris is well worth a mass. In 1598 he issued the Edict of Nantes, granting the Huguenots substantial rights as a grudging truce between the religions.
Catherine de' Medici was the widow of Henry II of France who became regent after the death of her son Francis II on the 5th of December 1560, ruling on behalf of the nine year old Charles IX. She tried to balance competing Catholic and Protestant factions and her January 1562 Edict of Saint-Germain was strongly opposed by the Guise faction.
The Edict of Nantes granted the Huguenots substantial rights but was later weakened during the 17th century. In October 1685, Henry IV's grandson Louis XIV issued the Edict of Fontainebleau, which formally revoked the Edict of Nantes and made the practice of Protestantism illegal in France.