March 9, 2020, became the first U.S. stock-market session halted by a modern circuit breaker during the COVID-19 market crash.
The S&P 500 fell 7 percent shortly after trading began, triggering a Level 1 market-wide circuit breaker and a 15-minute halt. The move occurred as investors reacted to the rapidly spreading COVID-19 pandemic and an oil-price shock caused by a breakdown in cooperation between Saudi Arabia and Russia.
The session was widely nicknamed Black Monday 2020. It was part of an exceptionally volatile period in which U.S. markets hit additional circuit breakers on March 12, March 16, and March 18. The S&P 500 ultimately reached a bear-market low on March 23.
Circuit breakers are intended to slow panic and give investors time to assess information. They were created after the 1987 crash, but the 2020 episode was the first time the U.S. market-wide mechanism had been triggered since its modern rules were established.