According to popular trading rules, May marks the beginning of a six-month period of unfavorable seasonality. This is often referred to as "Sell-In-May-and-Go-Away."
In theory, this would mean that better returns can be achieved by only owning stocks in the period from October through April and it's based on average performance results that are skewed lower by a few years in which the May-September period brought substantial downturns.
Truth be told, in any given year, actual market performance varies so widely that the "Sell in May" concept doesn't provide much of an advantage.
Jeffrey Hirsch, Editor-in-Chief, Stock Trader's Almanac, explains.