The most famous trading day of the 2015 Chinese stock-market crash was called Black Monday.
On August 24, 2015, China’s Shanghai Composite Index fell 8.49%, while major markets around the world also dropped sharply. The session followed a steep decline that had begun after the Shanghai market’s June 2015 peak. Concerns about China’s economic growth, currency policy, and the effectiveness of market-support measures unsettled investors.
The turbulence was not confined to one day. Chinese shares continued experiencing large swings, and the Shanghai Composite remained far below its June high for months. Global markets were affected because China had become an important part of the world economy and commodity-demand outlook.
“Black Monday” is also used for October 19, 1987, and for other market declines. In this question, it refers specifically to August 24, 2015, in the context of Chinese market turbulence.