Reuters/Stefan WermuthMay is here. And so of course we will now hear the old "sell in May and go away" cliche in markets a lot more often. Liz Ann Sonders, chief investment strategist at Charles Schwab, laid out the data underlying this saying in a post earlier this week: for the last half century, nearly all of the S&P 500's gains have happened between October and April. Sonders noted that the mean return from May to October was 1.3%. From November to April, however, the market has returned 7.1%, on average. And so returns during the summer are weak, not negative.