Although equities are flirting with new highs, history suggests the next few weeks could bring trouble.

September gets a bad rap since it’s historically the weakest month on the calendar. And that’s true, with the month posting an average loss of 1.2% since 1928. Even if we only look at periods of economic expansions, September is still a dud with an average loss of 0.6%.

But this analysis misses an important point — a point that most market pundits don’t make. And that is that the end of September is much weaker than the beginning of the month. Moreover, the weak performance tends to continue into October.

To be precise, an analysis of daily price returns for the S&P 500 going back more than 80 years shows that the weakest period of the year actually starts on September 9 and runs through the end of October. And that means that stocks — which are now “overbought” in the short term — will continue to fight serious headwinds in the days and weeks to come.

Check out the chart above, which compares the average daily returns of the S&P 500 between 1928 and 2009 to the year-to-date performance for 2010. As you can see, the historical performance did a nice job of calling the February turnaround, the April high, and the May swoon. But since then, the relationship has broken down.

For the month-to-date, stocks are performing significantly better than should be expected based on historical trends. This is like fighting the force of gravity — eventually, stocks will have to pullback to reflect the seasonal capital flows, investor sentiment, and other market forces that are determined by the calendar.

I’m looking for a tradable low sometime between October 7 and October 27. After that, stocks should enjoy a push up and over the April highs as the “pessimism bubble” that I recently wrote about bursts, the economy regains traction, and equities move higher at the expense of bonds.

The best advice for long-haul investors, hold off on new purchases for now. When the time is right, use this short-term correction as a buying opportunity. Stay tuned.

Be sure to check out Anthony’s new investment advisory service, the Edge, which is launching this month. He can be contacted at anthony.mirhaydari@live.com. Feel free to comment below.

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