Google informs me that the advice to "sell in May, and go away" comes from the tradition of British merchant bankers—I presume in the 19th century—to leave London for the country side in May and come back on St Leger's Day in September. I am partial to a good anecdote, but does it work? In order to check, I ran a little study using the S&P 500 going back to 1991. The first chart below shows the returns you would have foregone by selling in May and waiting 35 weeks and 17 weeks, respectively, before buying back. I have included both mean and median returns, because the outliers can skew the former when your sample size is not large. The second chart shows the results of a strategy which shorts the S&P 500 in May, buys the first week of October, and holds until year end.
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