October is, on average, a very good month for the U.S. stock market. But it is also a volatile month: Stock market veterans will recall the S&P 500 Index fell 20.5% on Monday 10/19/87 (28 years ago today), the largest single-day drop, in percent, in the history of the stock index. Keep in mind, in spite of tumbling 20.5% on the single trading day of Monday 10/19/87, the S&P 500 gained +5.3% for the entire 1987 calendar year.