AUG · ISSUE 36 · August 31, 2026
ZOOM INWall Street heads into its worst month
September is the only month with a negative average S&P 500 return in almost a century. Not a curse, not luck: statistics.
SEPTEMBER AVG
-1.1%
S&P 500 since 1928
YEARS POSITIVE
44%
only month < 50%
VIX TODAY
14.43
near 2026 lows
THE NUMBER
-1.1%
average S&P 500 return in September
Since 1928, September is the only month with a negative average return and the only one that closes higher less than half the time. Not a curse: a statistic.
DATA
THE NUMBER-1.1%
-1.1%
average S&P 500 return in September since 1928
It's the only month that, on average, loses money for the index across almost a century of history.
A statistical bias isn't a sell order: it's a reason not to walk in overconfident.
- AVERAGE
- — The mean of every September on record, not what this year will do.
- RETURN
- — How much the index rises or falls over the period, in percent.
QUOTE
PERSPECTIVEStatistics, not prophecy
“Seasonality is a bias, not a destiny: the worst Septembers arrived when the market was already weak, not mid-uptrend.”
The pattern is real, but each year's context matters more than the month on the calendar.
- BIAS
- — A statistical tilt that nudges, but doesn't dictate.
- TREND
- — The market's underlying direction over a long stretch.
HISTORY
MONTH BY MONTHThe month that breaks the pattern
Approximate historical figures since 1928. Almost every month is green on average. September isn't.
Monthly S&P 500 averages since 1928: September is the only one clearly in the red.
- MONTHLY AVG
- — The mean return of that month across nearly 100 years.
- ZERO LINE
- — The line between months that rise (green) and fall (red) on average.
HOW TO READ IT
3 KEYSWhat to do with September's stats
WHAT IT SAYS
September loses about 1.1% on average and closes higher less than half the years. No other month carries that record over a century.
WHY IT HAPPENS
There's no single cause. The end of summer, portfolio adjustments and tax selling all get cited, but none is decisive.
WHAT TO DO
Neither sell on the calendar nor ignore it. Use it to adjust expectations and position size, not to predict the month.
The pattern helps you calibrate expectations, not make the decision for you.
- POSITION SIZE
- — How much money you put at risk in a single trade.
- REBALANCING
- — Reworking positions to fit your risk to the moment.
ODDS
GREEN VS REDHow many Septembers finish green
Since 1950, September rises less than half the years. The only month below 50%.
Fewer than half. The only month of the year that fails this test.
- IN GREEN
- — The month closed with the index higher than it started.
- IN RED
- — The month closed with the index lower than it started.
WATCHLIST
5 REFERENCESFive names to watch in a weak month
| SPY | 769 | ▼ -0.3% | Tracks the S&P 500. Walking into its historically weakest month. |
| XLP | 84 | → +0.1% | Consumer staples. The classic defensive when the market turns cautious. |
| XLU | 84 | ▲ +0.2% | Utilities. A defensive sector with a steady dividend. |
| TLT | 88 | ▼ -0.4% | Long US bonds. A refuge if money seeks safety, but hurts when yields stay high. |
| VIXY | 10 | ▲ +1.0% | Volatility. Rises when market nerves pick up. |
Assets people tend to check when seasonality becomes a talking point. Not a recommendation, just examples.
- DEFENSIVE
- — A sector that holds up better when the economy or market weakens.
- ETF
- — A listed basket that tracks an index or sector.
WRAP-UP
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- SEASONALITY
- — A behavior pattern that repeats at certain times of year.
- S&P 500
- — Index of the 500 largest US listed companies.