Dating back to 1928, the S&P 500 and its precursor have declined 1.1% on average during September, making it the worst month of the year by a wide margin. The so-called "September Effect" hammered stocks once again this year. The S&P 500 has slipped 5.3% since the end of August. But history says the index is headed higher in the fourth quarter.

Here's what investors should know.

The fourth quarter has historically been the strongest quarter of the year for the S&P 500. Since 1928, the index returned an average of 0.6% in October, 0.9% in November, and 1.3% in December, for a total gain of 2.8% during the holiday quarter.

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Source Fool.com