September is the only month of the year with a long-term negative average return. Does that mean anything for investors?

Since 1928, the average move of the US stock market in September has been a decline of 1.17%. While other months finished in the red 39% of the time, for September it's 55%. Nine of the forty worst drops in S&P 500 history fall in this month.

The phenomenon is pronounced enough that Wall Street calls it the "September effect." The explanations include fund managers returning from vacation and rebalancing, hedge funds whose fiscal year ends in September harvesting tax losses, and apparently the very expectation of a weak month feeding on itself.

But the average hides an important detail. September 2025 gained 3.5% — the best September since 2010.

The more important number: $10,000 invested in the S&P 500 from 1996 to 2025 grew to $192,167. Missing just the ten best trading days left $85,490 — 56% less. Missing the thirty strongest days: $31,123.

The best and worst days cluster together. Selling to avoid September means you're just as likely to miss the recovery as the drop.

Is the September effect a real signal or just a pattern people keep finding because they're looking for it?