Which 1987 market crash led the Hong Kong Stock Exchange to close for four days?

The 1987 market crash that led the Hong Kong Stock Exchange to close for four days was Black Monday.

The Hong Kong market was closed from 20 to 23 October 1987 after severe international selling and market disruption. When trading resumed on 26 October, the Hang Seng Index fell 33.3%, one of the largest one-day declines recorded by a major stock index. The closure was intended to allow market participants and clearing systems to cope with extraordinary volatility.

The international episode began with the 19 October fall in the United States, but Asian and European markets were also heavily affected. Hong Kong’s concentration in property and financial shares increased the impact of the shock on local investors.

This question concerns the same worldwide event commonly called Black Monday, not a separate Hong Kong crash. The four-day closure is a distinctive fact about Hong Kong’s response to the 1987 turmoil.