In the digital asset ecosystem, September is often viewed as a treacherous period, a reputation fueled by lackluster historical data. Between 2013 and 2024, Bitcoin posted an average return of -3.5% during this month, including a particularly grim streak of six consecutive years in the red from 2017 to 2022. This phenomenon, dubbed "Septembear" by investors, extends beyond the realm of cryptocurrencies; traditional financial markets, such as the S&P 500, also display historically bearish seasonal trends at this time of year.
Several technical and structural factors explain this recurring sell-side pressure in the market. The return of market participants after the summer break is often accompanied by a resurgence in volatility, exacerbated by major events such as Federal Reserve meetings, quarterly derivatives expirations, and end-of-year accounting closures, particularly in Japan and the U.S. These events often drive tax optimization strategies, such as tax-loss harvesting, where investors offload losing positions to reduce their tax burden, which mechanically weighs on asset prices.
However, the relevance of this "curse" is increasingly being challenged, especially given the market's growing maturity. The last two years, 2023 and 2024, broke with tradition by posting positive returns. The overriding influence of the halving cycle seems to play a far more decisive role than the calendar alone. In accumulation phases, bullish momentum can easily override seasonal biases, demonstrating that Bitcoin's temporal dynamics are more closely correlated with monetary cycles than with random monthly fluctuations.
The integration of U.S. spot ETFs represents a major paradigm shift that undermines the seasonal signal theory. With the massive influx of institutional investors and corporations adding Bitcoin to their treasuries, marginal demand no longer follows the whims of the almanac. These capital flows, often automated and based on long-term portfolio rebalancing, neutralize old speculative reflexes. Consequently, indicators such as ETF inflows and derivatives open interest have become much more robust analytical tools than statistical observations derived from an era when the market was still in its infancy.