When the “Great Crash” came in 1929, it came in October. So, too, did the infamous “Crash of ‘87.” And last year, during a tortuous October that led to even lower lows in the months to come, the Standard & Poor’s 500 Index lost 19% of its value in just 30 days.

Investors can be excused if the word “October” is one that strikes fear into their hearts.

The trouble is, it’s actually September that deals investors the toughest monthly hands.

That’s September – as in the month that starts today (Tuesday).

After a rally that’s seen U.S. stocks surge 53% from their March lows (including 3.5% in August, alone), “investors are wondering if September will live up to its reputation as the month in which the S&P 500 posts its worst price performance and frequency of decline,” Sam Stovall, chief investment strategist at Standard & Poor’s Equity Research (NYSE: MHP), told MarketWatch.com yesterday (Monday).

Since 1929, September is actually the worst-performing months for stocks, with the S&P 500 suffering an average decline of 1.3% (compared to an average monthly advance of 0.5%), Stovall said.

The Dow Jones Industrial Average – the index that’s more closely followed by retail investors – tells a similar story. In fact, if you look at the Dow over the last 100, 50 and 20 years, September is the only month in which the average monthly performance has been negative, the Bespoke Investment Group concluded in a recent research report.

Over the past 100 years, the Dow has suffered an average decline of 0.96% in September, with a positive month 42% of the time. The average loss widened to 1.23% for the last 50 years and to 1.49% for the past 20.

Fall, in general, hasn’t been kind to investors: Of the 15 largest point declines in the Dow, six have come in October, four in September and two in November (See accompanying graphic).

Given that, investors “may have a reason to fear a setback in September,” Stovall told the news service.