Memorial Day marked the beginning of summer, and the old Wall Street adage "sell in May and go away" never seems to go away this time of year. Based on historical trends, investors do tend to sell in the month after Memorial Day. Since 2000, the Dow Jones Industrial Average has posted an average return of -1.03% in the one-month period following Memorial Day and the S&P 500 has posted an average return of -0.74%.