The October 1987 Hong Kong market crash was known as Black Monday. Hong Kong’s Hang Seng Index plunged, and the Stock Exchange of Hong Kong closed for four days from October 20 through October 23 to prevent disorderly trading.
The Hong Kong collapse was part of the worldwide Black Monday shock that began in October 1987. Markets in the United States, Europe, Australia, and Asia fell sharply within a short period, showing the growing interconnection of global finance.
Hong Kong’s market had already experienced substantial volatility after years of rapid growth and speculation. The temporary closure gave brokers and authorities time to assess unsettled trades and arrange a more orderly reopening.
The nickname is not unique to Hong Kong: “Black Monday” also describes the U.S. crash on October 19, 1987. The Hong Kong exchange’s four-day closure is the distinguishing fact in this question.