September Is Historically One of the Worst Months for the S&P 500. Here's 1 Move Not to Make.

Historically, September has been one of the weakest months for the S&P 500 (SNPINDEX: ^GSPC). From 1928 through 2023, the S&P 500 has averaged a negative return in September.

On top of history, there's plenty of current uncertainty swirling around as we enter September 2026, with the odds for interest rate hikes increasing, new trade disputes brewing between the U.S. and Canada, and the war between the U.S. and Iran reaching the six-month mark.

Here's what to expect -- at least from a recent history standpoint -- as well as one move to consider to turn any uncertainty into an investing advantage.

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The S&P 500's shaky performance in September

Given that the S&P 500 has averaged a negative return in September from 1928 through 2023, we can also look at more recent data to see timely trends. From 2017 through 2025, the S&P 500 averaged a return of -1.4% in September. What stands out is that, in September during those years, the S&P 500 has still been profitable 56% of the time.

That's possible because some of the losses during those years have been deep. For example, in September 2022, the S&P 500 sunk 9.3%, while its biggest gain was just 3.5% in September 2025. The odds that the S&P 500 will post a positive return in September are slightly better than a coin flip at 56%, but the magnitude of the potential losses can be jarring.

What history suggests doing next

While September can be a difficult month for investing, the good news is that over the long term, the S&P 500 tends to go up. From January 1996 to December 2025, for example, the average 30-year return was 10.4%.

Based on history, using the S&P 500 as a broad representation, one of the worst moves investors can make is selling their stocks during turbulent times. From 1996 to 2025, 48% of the best days for the S&P 500 were during a bear market, according to the Hartford Funds. And trying to time when to move in and out of stocks can be particularly costly. As a hypothetical example, if anyone invested $10,000 in the S&P 500 in 1996 and held it through 2025, that initial $10,000 would be worth $192,167. But just missing the 10 best days lowered that return potential by 56% to $85,490.

Instead of selling or trying to time the market, investors could consider setting up an automated dollar-cost averaging (DCA) plan. That way, an investor doesn't have to worry about buying at a peak by spreading out their purchases. Also, it's perfectly acceptable not to make a move at all if it doesn't fit into an investing budget at the time.

But if September ushers in a rocky month, the worst move to make, no matter the situation, is to panic-sell and move on from high-conviction stocks. The potential gains missed by selling are likely to outweigh the losses from what could be a short-term price drop.

Jack Delaney has no position in any of the stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. The Motley Fool has a disclosure policy.