ˌBlack ˈMonday BrE AmE
1 . Monday, 19 October 1987, the day on which ↑ share prices on the ↑ stock exchange suddenly fell by a large amount, and many people lost a lot of money
2 . Monday, 28 October 1929, the day on which share prices on the Stock Exchange suddenly fell by a large amount, leading to the ↑ Great Depression of the 1930s