US stock exchanges triggered circuit breakers four times during the March 2020 COVID-19 crash.
The market-wide trading halts occurred on March 9, March 12, March 16, and March 18, 2020. Each halt followed a sufficiently large decline in the S&P 500 under rules designed to slow panic selling and give market participants time to assess information. The first level of the modern system pauses trading after a 7% fall before 3:25 p.m. Eastern Time.
The crash reflected the economic shock of the COVID-19 pandemic, government restrictions, rapidly changing expectations, and an oil-price dispute between Saudi Arabia and Russia. The Dow Jones Industrial Average and other major indexes fell sharply before recovering as fiscal and monetary support expanded.
The four halts are sometimes confused with the number of major down days or with exchange-level pauses affecting individual shares. A market-wide circuit breaker is a specific mechanism applied to the broader US market, not every individual trading interruption.