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The Worst Month of the Year

September's reputation is deserved, but the reasons behind it are less certain.

Author

Jacob Radke
September 20, 2026 • Read Time: 1 minute

The Worst Month of the Year

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.

S&P 500 September price returns from 1950 through 2025 ranked from best to worst, with September 2026 month-to-date highlighted and the historical average shown as a dashed line.

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