Right now, the stock market rally's biggest threat might not be rising interest rates, mixed second-half guidance or nosebleed tech valuations.

It may simply be the calendar.

Over the past 15 years, August has proven to be the worst month for the

Dow

and

S&P 500

and the second-worst month for the

Nasdaq

, according to the Jeff Hirsch, an analyst at the Hirsch Organization, which publishes the

Stock Traders Almanac

.

"Volume dries up in August," he said. "Everyone's on vacation and there aren't a lot of people to support the market."

What's Worse

One reason that stocks fare so poorly in August is that investors are usually anticipating a bleak September, when tax-loss selling starts to kick in. Portfolio managers typically sell losing investments around this time to offset capital gains.

The tax year for most mutual funds ends on Oct. 31, while individuals have until Dec. 31 to neutralize their tax hit. But most investors make adjustments to their portfolios well in advance of those dates to get ahead of other sellers.

According to Hirsch, September has been the worst month for stocks over the last half-century. August's relatively recent stranglehold on the title is due in part to some specific events, including the 1998 Russian turmoil and the 1990 Kuwait invasion, as well as the higher concentration of people who take the month off, he said.

And things don't get much better in October. Although not as bad as many people think, October is still remembered as a month when big stock market crashes have occurred.

Amplification

So how seriously should you take these seasonal trends? John Bollinger, president of Bollinger Capital, thinks they could definitely have some impact on the market this year.

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"In consolidating markets, like the one we're in now, seasonality becomes more important because there is no overarching bullish trend to bail people out," he said. "It's certainly a good time to be cautious and hedge where appropriate."

Hirsch agrees and said he is looking for stocks to pull back over the next eight weeks.

"I don't see us making a new low. I see us giving back another 1000 Dow points at most and then having a strong finish," he said. Although there are few positive catalysts to support the market in the near term, stocks typically pick up steam in November and December.

Tom McManus, chief investment strategist at Banc of America, isn't concerned about a near-term correction. He notes that although stocks did fall in September of last year, August was still a pretty good time to get into the market. The S&P 500 is up almost 7% from where it was at the end of July last year.

Jeff Matthews, president of Ram Partners and a contributor to

TheStreet.com's

sister site

Street Insight

, also noted that there have been a few times in history when stocks did very well in August.

"One of the great bull markets of all time started in August 1982, so if you were sitting there saying August is a terrible month you would have missed huge moves in the market," he said.

Indeed, stocks rose 11.5% that month and were up 9.8% in August 1984 as bear markets came to an end. The month of September has also been good to investors on several occasions, including 1995, 1996, 1997 and 1998, with average gains of 4.2%. Meanwhile, October is known as the "bear killer" because it helped to turn the tide in nine bear markets, according to the

Almanac

.

"I don't pay attention to seasonal voodoo," Matthews said. "What I think investors should pay attention to is the rise in bond yields."

Alternatives

The 10-year note's price has fallen roughly 10% over the past six weeks in one of the largest declines for any such period since 1980, according to David Joy, capital markets strategist at American Express Financial Advisors.

Analysts say equity valuations are becoming increasingly stretched in the face of rising long-term interest rates.

Indeed, Richard Bernstein, chief quantitative strategist at Merrill Lynch, said his dividend discount model now suggests that stocks are about 15% to 20% overvalued. The proportion of S&P 500 stocks with dividend yields greater than the 10-year note has also plummeted, he said.

"Cash is not trash anymore," said Matthews. "That's what's really changed the landscape to the detriment of stocks."