The stock market's history of September swoons: By the numbers

Historically, the stock market performs most miserably in September (no one is quite sure why), and this month doesn't seem likely to reverse the annual trend. The S&P 500 drooped 3 percent last Thursday and Friday — the first two days of the month — and slipped another 0.7 percent Tuesday. "If history is any guide, for it's never gospel, we may be in for another rough ride," says Standard & Poors' Sam Stovall. Here, a brief guide, by the numbers, to the dreaded September swoon:

0.8

Average percentage that the S&P 500 has lost each September, from 1950 to 2009, making it the worst month for the index. September is also the worst month for the Dow and Nasdaq.

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1.3

Average percentage that the Dow has lost each September since 1929

10

Percentage decline suffered by the Dow in September 1929, precipitating the Great Crash

30.7

Percentage decline suffered by the Dow in September 1931 — still the worst month ever for stocks

777.68

Number of points the Dow cratered on Sept. 29, 2008 — the largest single-day drop in the index's history

684.81

Number of points the Dow lost on Sept. 17, 2001 — the day the stock markets reopened after the 9/11 attacks. It was the third-largest single-day drop in the index's history, and followed the longest market closure since the Great Depression.

1,102.19

Number of points the Dow dropped in September 2001 — the fourth-worst month ever for the index

1,071.57

Number of points the Dow shed in September 2002 — the fifth-worst month ever for the index

5

Number of times in the past decade that the S&P 500 has dropped at least 5 percent in September

4

Percentage that the S&P 500 gained last September in a welcome twist. Not only did "the highly anticipated September swoon never come to fruition," says Steven Russolillo in The Wall Street Journal, "but the stock market surged by historic proportions." The Dow gained 8 percent in September 2010.

Sources: Bloomberg, Dave Manuel, MSNBC, Smart Money, Reuters, Wall Street Journal