April Fool's Day in the 1600s & 1700s

Escape of the Duke of Lorraine (1634) During the Thirty Years' War, the Duke of Lorraine and his wife were held as prisoners within the walls of Nancy, the capital city of Lorraine, at the orders of the King of France. But on April 1, 1634, the Duke and Duchess escaped by disguising themselves as peasants and walking out through the front gate of the city. Their escape was almost foiled when a peasant recognized them and ran to tell a soldier, who communicated the news to his commanding officer. However, it being April 1st, the officer thought he was being made an April Fool (or a Poisson d'Avril, as the French say) and didn't believe the report. By the time the French realized the report of the escape wasn't a joke, the Duke and Duchess were too far away to be overtaken.

Bickerstaff’s Predictions (1708)
In early 1708, a previously unknown London astrologer named Isaac Bickerstaff published an almanac in which he predicted the death by fever of the famous rival astrologer John Partridge on March 29 of that year. Partridge indignantly denied the prediction, but on March 30 Bickerstaff released a pamphlet announcing that he had been correct: Partridge was dead. It took a day for the news to settle in, but soon everyone had heard of the astrologer's demise. And so, on April 1st the joke came to full fruition (suggesting it was a deliberate April Fool joke) when Partridge was woken by a sexton outside his window wanting to know if there were any orders for his funeral sermon. Bickerstaff, it turned out, was a pseudonym for the satirist Jonathan Swift. His prognosticatory prank worked so well that Partridge was eventually forced to stop publishing almanacs, unable to shake his reputation as the man whose death had been foretold.