What was Black Saturday?

Black Saturday refers to Saturday, September 7, 1940, during World War II. On that day, over three hundred German bombers and six hundred fighter planes attacked the city of London in a day-long raid. Over one thousand bombs and incendiary devices were dropped on London on this day. Most of the attack was focused on London's Docklands. While their targets may have been military ones, there was no concern taken for the hundreds of thousands of civilians in the area. This raid marked the beginning of the London Blitz. An estimated five hundred people were killed as a result of the raid that day.
The term Black Saturday may refer to the color of the sky, which was turned from blue to black as a result of the fires of burning London.
The Blitz marked a period in which the German high command decided to focus its strategy on bombing London and other English cities, in order to crush English morale. Previously, the German Luftwaffe had been battling the Royal Air Force for months, in order to achieve air superiority over England. This was to the prelude to a planned invasion of the country. However, by September, the Germans had made little progress and began turning their attention towards attacking cities. They opened up this new strategy with the Black Saturday attack. The attack caught most people in England by surprise, especially since a concentrated attack on cities had not occurred before. Instead of splitting up to bomb military targets, the German bombers stayed in tight formation and bombed London for hours.
The Blitz that started with Black Saturday lasted until the spring of 1941 when the Germans shifted their focus away from Great Britain and toward the invasion of the Soviet Union.
https://www.theatlantic.com/magazine/archive/2008/04/black-saturday/306706/