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
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
the day after Whit Sunday, which used to be a public holiday in the UK. There is now a holiday around the same time called the "Spring Bank Holiday", but it is not always on Whit Monday.