US stocks and cryptocurrencies begin September facing their historically weakest month. The S&P 500 closed August with gains exceeding 2% and set record highs, while Bitcoin gained 24.95% and Ethereum advanced 32.5% during the month.
Stocks' September Pattern
Historical data reveals a consistent seasonal weakness in equities. The Dow Jones Industrial Average has fallen an average of 0.8% in September since 1950, according to the Stock Trader's Almanac. The S&P 500 has declined 0.7% over the same period, while the Nasdaq Composite has dropped 0.9% since 1971 and the Russell 2000 has lost 0.8% since 1979.
Bank of America data extending back to 1928 show an average September S&P 500 loss of 1.17%, with the index falling in 56% of those years. Almanac authors attribute the pattern to portfolio managers cleaning house after the summer break.
Crypto's Calendar Weakness
Digital assets display similar seasonality. Bitcoin has averaged a 2.87% decline in September since 2013, marking its weakest month, with a median loss of 2.44%. Ethereum shows a wider gap, with an average September drop of 9.40% since 2015 and a median decline of 9.14%.
Recent Exceptions to the Pattern
However, recent years have deviated from historical norms. Bitcoin closed higher in each of the past three Septembers, gaining 5.16% in 2025 and 7.29% in 2024. Ethereum rose 3.20% in September 2024. Similarly, the S&P 500 rose 2.02% in September 2024 and 3.5% in 2025.
Current Market Context
Both equities and crypto enter September with significant momentum following August's strong performances. The backdrop includes midterm election year volatility patterns, inflation running at 3.7% in July personal consumption expenditures—nearly double the Federal Reserve's 2% target—and elevated crude oil prices.
