In 1922, T.S. Eliot called April the cruelest – actually, "cruellest" – month. With the benefit of hindsight, we can safely say he should have picked October.
Stock market lore has labeled just four days "Black," and they all happened in October: Black Thursday (October 24, 1929), Black Monday (October 28, 1929), Black Tuesday (October 29, 1929) and the other Black Monday (October 19, 1987). The most stomach-wrenching drop of 2008 happened when Lehman Brothers folded on September 15, but we threw the following October in anyway, because 14% down in a month is nothing to sneeze at.
The chart above shows the cumulative change in the daily closing price of the Dow Jones Industrial Average – stupid it may be, but it was the only game in town in 1929 – over the course of each of October's 22 or 23 trading days. For reference, Black Thursday was the 18th trading day of October 1929; the Dow closed down only 2% because New York Stock Exchange bigwig Richard Whitney placed a string of massive buy orders to stem an 11% intraday loss. No such luck the following Monday (the 20th trading day) and Tuesday (the 21st), when the Dow shed 14% and 12%, respectively. Black Monday 1987, which saw the Dow plummet by a record 23%, was the 13th (yup) trading day of the month.
So far October 2017 seems like it will pass uneventfully. But there's still time.