This week was the 28th anniversary of 1987’s Black Monday, a reminder that large declines are a feature of equity markets. There’s a 1-in-26,238 chance the U.S. stock market over the next 24 hours will plummet as much as it did on Black Monday, the day of the 1987 crash. According to Mark Hulbert, a single-session drop as big as the one in 1987, for example, is predicted — over long periods of time — to occur once every 104 years, on average. So it’s possible we’ll get two 20% crashes in our lifetimes — or none.