September is fewer than three weeks away. Feeling nervous? Maybe you should be. For investors, the period between Labor Day and Halloween is proving an annual fright show. And no one knows why.

It was, of course, in September last year that Lehman collapsed and everything fell apart. But then it was also September-October 2002 that the last bear market plunged to its lows.

The 1998 financial crisis? It began late August, and rolled on for two months.

The famous crash of 1987 came in October. But most people have forgotten that the market actually started sliding downhill in late August.

That’s almost exactly what happened in 1929 too. The big crash came in October, but the market peaked just after Labor Day. Prices began falling through September, then tumbled further still.

The worst month of the Depression? September, 1931, when the Dow fell about 30 percent. It was also in September, 2000, that the bear market really got going.

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