Bitcoin (BTC) Enters Historically Weak September With 2.87% Average Monthly Loss

Wall Street’s Weakest Calendar Month

US equity and digital-asset markets enter September carrying the weight of the statistically weakest month on the calendar. The S&P 500 finished August with gains of more than 2% and set record highs along the way — closing at 7,798.99 on August 13, its 27th record close of 2026 — while the Dow Jones Industrial Average added more than 1%. Yet the same month holds a long losing record for stocks. Since 1950 the Dow has fallen an average of 0.8% in September, and the S&P 500 has shed 0.7% over the same span, according to the 2026 Stock Trader’s Almanac. The Nasdaq Composite has dropped an average of 0.9% since 1971, and the small-cap Russell 2000 has lost 0.8% since 1979. Bank of America data stretching back to 1928 point to an average September loss of 1.17% for the S&P 500, with the index closing lower in 56% of those years.

Almanac authors Jeffrey Hirsch and Christopher Mistal tie the pattern to fund behavior after the summer break: portfolio managers returning after Labor Day tend to “clean house” in September, a rebalancing rhythm that drains demand precisely when positioning is thinnest. August’s strength was earnings-driven — pre-tax corporate profits reached $4.8 trillion in the second quarter, the highest share since at least 1950 — and investors gaining large-cap tech exposure through vehicles such as the Nasdaq-100 ETF rode the advance. Several pressures now sit against that backdrop. The United States and Iran exchanged strikes for the first time in over a month, July personal consumption expenditures inflation ran at 3.7% — nearly double the Federal Reserve’s 2% target — and crude prices have surged. Roth chief technical strategist JC O’Hara adds that midterm election years have historically produced elevated volatility in September and October, a cycle signal that aligns with the seasonal drag.

Crypto’s Own September Record

Digital assets carry a matching seasonal profile. Monthly-returns data on Coinglass shows Bitcoin has averaged a 2.87% decline in September since 2013 — its weakest month of the year — with a median loss of 2.44%. Ethereum shows a wider gap: the asset has averaged a 9.40% September drop since 2015, with a median decline of 9.14%, according to CryptoRank analytics. The effect also propagates down the market-cap curve, where large-cap altcoins such as Toncoin (TON) trade on thinner liquidity and tend to amplify the majors’ swings rather than cushion them. September weakness in crypto has often mirrored the same post-summer liquidity drain that hits equities, compounded by derivatives positioning resets ahead of the fourth quarter and thinner volumes across crypto exchanges.

The recent record complicates the bearish reading, however. Bitcoin closed higher in each of the past three Septembers, gaining 5.16% in 2025 and 7.29% in 2024, and Ethereum rose 3.20% in September 2024. Equity seasonality has softened in parallel: the S&P 500 rose 2.02% in September 2024 and 3.5% in 2025, back-to-back gains that cut against the long-run average. Both assets also enter this September with strong momentum — Bitcoin gained 24.95% in August, while Ethereum advanced 32.5% — a breadth of strength that historically has not needed the calendar’s permission. Traders should also note that funding-rate resets in early September have historically thinned the leverage cushion, making sharp intramonth wicks more likely even when the monthly close ends green. The core question is whether momentum absorbs the seasonal effect, or the pattern reasserts itself after a three-year hiatus. Readers tracking the market in real time can follow live spot and futures prices on Bybit.

Whether Seasonality Wins Again

Our desk’s reading: this is a collision between a statistical headwind and unusual momentum, and the primary record — the Coinglass monthly-returns dataset — shows the September skew is real but has failed to bind in three straight years. With Bitcoin spot near $78,650 and Ethereum at $2,472.85 as of this writing, the market shows no panic pricing. Capital seeking ballast through the seasonal window has historically rotated toward gold or a gold-backed token such as Tether Gold (XAUT), and investors sizing entries into thinner books can compare venue depth in our guide to the best crypto exchanges. A decisive break of August’s trend would mark seasonality’s return; continued strength would further erode the pattern’s predictive pull.