Markets Won't Wait for September

But history also shows August can be a very positive month for investors, at least during election years. Since 1900, August has produced an average return of 2.7% for the S&P 500, making it the best performing month by a wide margin, according to S&P.

"While many investors will be spending the month of August at the beach, the financial markets will not necessarily wait for vacations to end and the kids to be back in school before making a material move," Scott Wren, senior equity strategist at Wells Fargo's (NYSE:WFC) Wells Fargo Advisors, wrote in a note.

The best August occurred in 1982 when the Paul Volcker-led Federal Reserve started to cut interest rates, kicking off the secular bull market that lasted until 2000.

"There's a chance we could have some turbulence time in August," said Sam Stovall, chief investment strategist at S&P Capital IQ. "Don't allow your emotions to get the better of your portfolio. Positive things can happen and as a result go with the flow.