RESEARCH
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The Worst Month of the Year
September has been the S&P 500's worst month since 1950, averaging a 0.6% decline while the other 11 months averaged a 0.88% gain.
Jacob Radke
1 min read
S&P 500 September price returns, 1950–2025, ranked; September 2026 month-to-date through September 17.
Source · Financial Modeling Prep; Fjell Capital analysis
Average September
−0.6%
Average S&P 500 price return in September from 1950 through 2025, the worst of any calendar month.
Average other month
+0.88%
Average S&P 500 price return across the other 11 calendar months over the same period.
September 2026 so far
−0.7%
S&P 500 price return for September 2026 through September 17.
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Markets & Investing
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Key takeaways
September averaged a 0.6% decline for the S&P 500 from 1950 through 2025, the worst of any month.
The other 11 months averaged a 0.88% gain.
There is no single accepted explanation; fund fiscal year-ends, quarter-end rebalancing and post-summer repositioning are possible factors.
September 2026 was down 0.7% through September 17, close to the long-run average.
Historically September has been the worst month of the year for the S&P 500.
From 1950 through 2025, it averaged a 0.6% decline. The other 11 months averaged a 0.88% gain.
There is no clear explanation for why. Possible reasons include mutual fund fiscal year-end activity, quarter-end rebalancing and investors returning from summer vacations and repositioning their portfolios.
So far, 2026 is down 0.7% through September 17, almost exactly in line with the long-run average.
Until next week,
Jacob
Sources & method
S&P 500 monthly price returns: Financial Modeling Prep daily ^GSPC closes, 1950 through September 17, 2026; Fjell Capital calculations. FMP API documentation.
Possible drivers of September weakness: Nasdaq Global Indexes, 2025.
Mutual fund fiscal year: Internal Revenue Service, Instructions for Form 8613.
Written by
Jacob Radke
Partner, Wealth Advisor
Jacob writes the weekly charts. Jacob leads the firm’s platform, including trading, research, portfolio systems and technology. He also sits on the firm’s board and investment committee, and advises client families.
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