September Has Been One of the S&P 500’s Weakest Months. What History Says About Q4

Although September carries the worst reputation on Wall Street, the market this month has quietly refused to cooperate with the folklore. The benchmark S&P 500, proxied through SPDR S&P 500 ETF Trust (NYSEARCA:SPY), has advanced 0.82% from Sept. 1 through a.m. trading on Sept. 28, trading around $768. Year to date, the fund is up 12.38% as of Monday morning, Sept. 28, and its one-year return sits at 15.72%.

However, the reason September gets talked about at all shows up when you widen the lens to the last quarter century, and the reason Q4 matters shows up right after that.

What September Has Actually Done Since 2000

Here is the seasonal record with its sample period stated plainly, because the starting year changes the claim. Across the 26 completed Septembers from 2000 through 2025 in the SPY monthly adjusted series, nine closed below their August adjusted close: 2000, 2001, 2002, 2008, 2011, 2015, 2020, 2022 and 2023. That is roughly one September in three finishing red versus the prior month in the sample the data actually supports. It is a real pattern. It is also a small sample, and small samples deserve small confidence intervals.

What stands out is how sharply negative September can be when it does turn down. September 2008 fell from an August adjusted close of $92.35 to $83.64. September 2002 dropped from $59.05 to $52.86. September 2022 slid from $373.61 to $339.08. Those are the episodes doing most of the reputational damage.

Q4 After a Weak September Follows a Mixed Record

Now to the part the retirement-account reader actually cares about. What happened next? Across those nine weak Septembers, October, November, and December did not follow a single script. In 2001, 2011, 2015, 2020 and 2023, October or November rebounded and December generally held. In 2022, October ran to $366.64 and November to $387.02 before December gave some back at $364.74. In 2002, October and November recovered but December closed lower again at $57.30.

Then the two years that broke the pattern. In 2008, September weakness rolled straight through October at $69.82 and November at $64.96. In 2000, October, November, and December each stepped lower, ending at $82.81. Those two failures traced directly to a credit crisis and the unwinding of the dot-com valuation regime. The pattern failed because the mechanism driving prices was no longer a calendar quirk.

Why This Year’s Setup Differs

Seasonality is only a pattern, and the underlying mechanism is what decides Q4. Two indicators frame the current setup. The 10-year Treasury yield opened at 5.196% on Sept. 28, the high of the trailing year and a 99.6 percentile rank in the one-year distribution. That is a genuine headwind for equity valuations. On the other side, the CBOE Volatility Index printed 15.9 on Sept. 28, just above the guide’s 15 threshold for low volatility and near the one-year low of 14.21 on Sept. 22. Complacent options positioning, in other words, into a rate environment that has been anything but calm.

Consumer sentiment adds a third data point. The University of Michigan index registered 55.2 in July 2026, an improvement of 11.5% from the prior month but still under the source’s 60 recessionary-level threshold. Rising rates, cheap volatility, and a cautious consumer rhymes more with the years where seasonality was the story only until macro took the microphone, rather than the profile that produced 2001 or 2015.

What the 401(k) Saver Should Actually Do

Nothing. That is the committed view. A workplace retirement account is a decades-long dollar-cost-averaging vehicle, and the seasonal record from 2000 through 2025 does not justify a tactical move for a saver whose withdrawal date is 10, 20, or 30 years out. Nine weak Septembers in 26 years, followed by a Q4 pattern that has both recovered and failed, is better treated as a footnote you keep in mind so October headlines do not push you into changing an allocation you set for reasons that have nothing to do with the calendar, rather than a signal you trade a payroll contribution around.

The historical mirror worth carrying into Q4 is narrower. When September weakness ran through the fourth quarter, in 2000 and 2008, an identifiable macro rupture was already underway. The current backdrop has stresses worth watching, particularly the 5.11% 10-year yield, but no rupture in the market yet. For readers close to their withdrawal date, a bad tape in the first few years does far more damage than the same drawdown later, which is the whole subject of our free early retirement defense guide: The First Five Years. Long term, Wall Street still heads higher in the decades to come, and a retirement saver’s job in the next 90 days is to keep contributing and let the pattern remain just a pattern.

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