Since many people taking summer vacation, the stock market tends to have less activity. This means not much of the demand. Stocks go up when demand is higher than supply.
‘Sell In May And Go Away’: historically stocks are underperformed in May and ending in October, compared to the six-month period from November to April. American traders who are likely to spend more time on vacation between Memorial Day and Labor Day mimic this trend and have adopted the phrase as an investing adage.