Black Monday was the Monday after Easter on 13 April 1360, during the Hundred Years’ War (1337 – 1360). The Hundred Years’ War began in 1337; by 1359, King Edward III of England was actively attempting to conquer France. In October, he took a massive force across the English Channel to Calais. The French refused to engage in direct fights and stayed behind protective walls throughout the winter, while Edward pillaged the countryside. By 13th April he had sacked and burned the suburbs of Paris and was now besieging the town of Chartres.
At nightfall, a sudden storm came upon Edward’s troops, who were camped outside Chartres. Unfortunately, for Edward, their tents provided little protection. The temperature dropped. Lightning. Freezing rain. High winds. Hailstorms. Many of the soldiers abandoned the encampment. 1000 English soldiers and some 6000 horses were killed by the intense hail storm. Horses also fell to the storm; many stampeded. The casualties were larger than any previous battle. Two of the English commanders met their death. King Edward was on his knees begging for God’s mercy.
The carnage was described as “a foul day, full of myst and hayle, so that men dyed on horseback [sic].”
From the contemporary French Chronicle of Jean Froissart:
... for an accident befell [Edward III] and all his army, who were then before Chartres, that much humbled him, and bent his courage.
During the time that the French commissioners were passing backwards and forwards from the king to his council, and unable to obtain any favourable answer to their offers, there happened such a storm and violent tempest of thunder and hail, which fell on the English army, that it seemed as if the world was come to an end. The hailstones were so large as to kill men and beasts, and the boldest were frightened.
The king turned himself towards the church of Our Lady at Chartres, and religiously vowed to the Virgin, as he has since confessed, that he would accept of terms of peace. He was at this time lodged in a small village, near Charters, called Bretigny; and there were then committed to writing, certain rules and ordinances for peace, upon which the following articles were drawn out.
France after the Treaty of Brétigny – French territory in green, English territory in pink John Richard Green – Taken from History of the English People, Volume 2 Transferred from en.wikipedia to Commons ~ Public Domain
Edward rushed to pursue peace with the French and as a direct result of the killer storm, on May 8, 1360, the Treaty of Bretigny was signed. By this treaty Edward agreed to renounce his claim to the throne of France in return for sovereignty over Aquitaine and Calais. The French agreed to pay a handsome ransom for the release of their king John II who was held captive in England.
Fighting resumed nine years later, when the king of France declared war, claiming Edward had not honored the treaty. The last phase of the Hundred Years’ War did not end until 1453.
The legacy was mentioned in Shakespeare:
“It was not for nothing that my nose fell a- bleeding on Black Monday last, at six o’clock i’ the morning.” —Shakespeare: Merchant of Venice, ii. 5.
Sources:
Encyclopedia of the Hundred Years War (Google Book) by John A. Wagner
Every Woman Dreams... 
Yes, powerful men pillage innocent civilians in their homes and when nature retaliates, it’s considered a tragedy. Enough so that the day gets a commemorative title. Even though it brought on a temporary truce. If only nature intervened more often.
I recognize the story. Thea Beckman (1923-2004) wrote a Dutch trilogy (fiction based on facts) about the Hundred Years War for children/adolescents in the late 70’s of the past century. I really loved her books and because of her books I learned about this part of English and French history.
In 2020 I read the books again (still on my bookshelf) and searched for and found more about this war. It’s great to read your post with -for me- new information. Thank you.