Wikipedia:Selected anniversaries/April 13
- 1204 - Fourth Crusade: Forces under Boniface the Marquess of Montferrat and Enrico Dandolo the Doge of Venice captured Constantinople, capital of the Byzantine Empire.
- 1598 - King Henry IV of France (pictured) issued the Edict of Nantes, allowing freedom of religion to the Huguenots.
- 1860 - The Pony Express, the first mail service across the North American continent from the Missouri River to the Pacific coast, was successfully completed for the first time.
- 1919 - British and Gurkha troops opened fire on a peaceful political gathering in Amritsar, Punjab in India, killing hundreds of Sikhs, Hindus and Muslims.
- 1943 - World War II: Germany announced the discovery of a mass grave of Polish prisoners-of-war executed by Soviet forces in the Katyn Forest Massacre.