The S&P 500 is up 3.4 percent so far this month and on pace for the best August performance since the year 2000.
But here comes September, and if history is a guide, it may not be good news for investors.
Since 1950, September is the worst performing month for the index, losing on average more than negative 0.5 percent.
If you look just at the last 20 years, the negative trend has continued as September was the second worst performer, and as a result, second most volatile month.
Average volatility as measured by the VIX spiked up by more than 8.65 percent during the month.
After the financial crisis, the negative trend changed, and the S&P was up four of the last five years in September for an average gain of 2.2 percent. But what's troubling for stocks is the performance of September markets, after a powerful move higher in August.