Weak September Seasonals Precede Strong Midterm Trends ​ ​ George Smith | Portfolio StrategistSeptember 01, 2026  Stocks have enjoyed a powerful run off the spring lows and have largely shrugged off concerns around growth, inflation, higher interest rates, the effects of artificial intelligence (AI), geopolitics, and policy uncertainty. As the calendar turns to September, however, they are entering what has historically been, from a seasonality perspective, the most challenging month of the year for equities. So why does seasonality exist at all? A handful of recurring, calendar-driven behaviors tend to leave a footprint on returns: tax-related selling, mutual fund fiscal year-ends, corporate buyback windows, summer liquidity lulls, and the predictable rhythm of earnings season all nudge markets in loosely repeatable ways. That is why these patterns are worth watching; when a behavioral tendency persists across decades, it can offer a useful sense of the prevailing wind. From a seasonality perspective, September has consistently stood out. Since 1950, it is the only month with a negative average return, at roughly -0.6%, and the S&P 500 has finished the month higher less than half the time. That weakness has been even more pronounced over the past five and 10 years, so the seasonal caution flag is a fair one to raise. Performance prior to 1957 is measured by the predecessor index, the S&P 90. Past performance does not guarantee future results. Seasonality Trends Weaker for Equities in September