September's Scary Average Is a Numbers Trick, Not a Forecast: Why This Year's Strong Entry Changes the Setup

September is the month every calendar-watcher dreads with reason: since 1926 the S&P 500 has averaged a -0.7% decline in September, the only calendar month with a negative average return. Ryan Detrick, chief market strategist at Carson Group, calls it the worst month on average since 1950. At face value, that is a tidy reason to lighten up before the calendar flips. It is also, if you look at the actual distribution rather than the headline mean, a thinner case than it appears — and the market's entry into September 2026 does not look like the setups where the seasonal drag historically bit. Start with the arithmetic, because the fear and the typical experience here are not the same statistic. Fisher Investments, which dug through the century of data, points out that the negative average is almost entirely the work of a few catastrophic Septembers: 1931's -29.6%, 1937's -13.8%, 1974's -11.5%, 2002's -10.9%, and 2022's -9.2%. Strip those "epically bad Septembers" out and the leftover average is roughly flat. The median September actually rounds to +0.1%, and the index has risen in 52% of Septembers since 1925. A strong average of -0.7% sitting on top of a positive median means the "curse" is a distribution problem, not a calendar law.

What set those five Septembers apart was not the month on the page. Each had an outside driver attached to it: a banking collapse and the pound leaving gold in 1931, the oil shock and inflation mess of 1974, the late-cycle rate tightening of 2022. Fisher's conclusion is blunt: "stocks didn't fall because it was September." Recent benign Septembers reinforce the point — the index rose 8.9% in 2010, 2.1% in 2024 and 3.6% in 2025 — while the down years clustered around genuine economic shocks.

Selected historical September returns for the S&P 500Benign Septembers versus crisis Septembers · calendar-month total return

Recent Septembers were strongly positive (+2.1% to +8.9%), while crisis Septembers fell sharply (-9.2% and -10.9%), contrasting the benign recent pattern against tail-driven losses.

Period	September total return (%)
Sep 2010	8.9
Sep 2025	3.6
Sep 2024	2.1
Sep 2022	-9.2
Sep 2002	-10.9

That still leaves the honest question half-answered: a positive median is not a green light, because the month is not harmless in every year. Detrick's work supplies the missing condition. He argues September's bad reputation is largely earned in years the market enters the month weakly — the setup that lets a small seasonal wobble turn into a real drawdown. Enter with strength, on his read, and the historical pattern has been far kinder. So the question becomes a live, checkable one: what state does the market actually enter September 2026 in? This is where the current data comes in, and it points the same direction. As of September 6, Ainvest data shows SPY, the S&P 500 exchange-traded fund, at roughly 770 against a 52-week high of 779 — trading just off its peak — up about 12.9% year to date, with an RSI reading near 56 and price sitting comfortably above both its 50-day average (about 757) and its 200-day average (about 712). Detrick's snapshot corroborates the strength from the breadth side: nearly 70% of S&P 500 stocks are trading above their 200-day moving average, and the VIX, the market's fear gauge, sits near 14.5 — well inside low-volatility territory. That is the opposite of a market limping into the month; Detrick's own summary is that the "calendar looks worse than the market." The analytical operation here is the same one I run on any single-name rating: separate the factor's level from its trajectory, then ask whether the current read still supports the thesis. A static calendar rule — "September is down on average, so sell" — treats a mean that a handful of crisis years produced as if it applied to every September. It does not. The version of the signal that has actually survived is conditional: it bites when the market enters weak, and it has repeatedly not when the market enters strong. Today's entry checks the boxes on the strong side on both price and breadth. The read this leaves is not a prediction of what September 2026 will return; no dataset in this evidence projects that, and I am not going to pretend one does. It is a judgment about what the calendar alone is and is not telling you. On the evidence, the historical pattern does not support de-risking the whole market on the season itself, and it does support holding through the month when the entry is this healthy. What changes the call are the conditions that turned prior benign Septembers ugly: breadth sliding toward 50%, the VIX breaking above 20, or SPY closing back under its 200-day average. None of those describe the tape today. Until one of them does, the professional move here is the unglamorous one — sit tight and let the setup, not the month, make the decision.