September Was the Only Month With a Negative Average Broad-Market Return, 1926-2025

For the 100-year period ending in 2025, September alone posted a negative mean in this cap-weighted U.S. stock series; November topped both the mean and median rankings.

The comparison runs through Dec. 31, 2025. September’s median return was positive, but the lowest among the 12 months. October ranked as the second-weakest month, while July and December were also among the stronger months.

A separate S&P 500 comparison using data as of Sept. 16, 2026, found an average September return of negative 1.1% since 1928, with positive returns in 45% of those months. Its benchmark and start date differ from the broader-market series, so the figures are not directly comparable.

The historical record gives some support to the “sell in May and go away” pattern: The six months from November through April had higher average and median returns than the six months from May through October. Still, May through October averaged nearly 4% over the six-month period, with July contributing disproportionately.

A separate total-market index fell 0.76% in September 2026, outside the 1926-2025 historical comparison. Strategist Dan Lefkovitz cautioned against reading too much into monthly patterns: “I wouldn’t make too much out of monthly return patterns.”