Stock traders are preparing for a challenging September as historical data suggests it has been the weakest month for U.S. equities. The SPDR S&P 500 ETF Trust (NYSE: SPY) has seen average losses of 0.98% over the last 20 years, 0.96% over 30 years, and 1.04% over 40 years during September. In the last five Septembers, the index has only closed higher once, with an average monthly drop of 1.4%. Analysts such as Adam Turnquist from LPL Financial and Paul Ciana from Bank of America have indicated that September has been consistently bad for the U.S. stock market. Since 1928, the S&P 500 has declined in September 56% of the time with an average drop of 1.17%. Despite the S&P 500 reaching a new all-time high in August, historical trends suggest a challenging September ahead for the market.