So, if the stock market crashes in 2020, the best thing you can do is to take a deep breath and keep your eye on the long term. Motley Fool Returns. Stock Advisor S&P 500. 325% 77%. 5 Bold Predictions for the Stock Market in 2020 Some scenarios that could play out over the next year. there are some major catalysts that could spark the first U.S. recession in more than a Stock Market Crash of 1987. On Monday, October 19, 1987, now known as Black Monday, the Dow Jone Industrial Average fell 23% in one day.. This crash happened on an ordinary day without any significant news. At the time, this was the largest single-day percentage decline.. When the crash first happened, prominent economists, predicted we are on the cusp of another Great Depression.