Indeed, since the end of World War II, the S&P 500 index of blue-chip U.S. stocks has averaged losses in only three months of the year: February, August, and September. And over time, August and September have been the worst two months for equity market returns. Since 1945, the S&P 500 has lost 0.2% on average in August and 0.7% in September. What’s more, the second-worst ever monthly drop in equities took place in an August — back in 1998, when the S&P sank nearly 15% in just four weeks.