September’s Seasonality: The Market’s Weakest Month 📉
August 27, 2025
Today's number is... 9
We’re about to enter month 9 — September, which is the worst month on the calendar for the S&P 500.
Here are the charts:
Let's break down what the charts show:
Monthly Seasonality: The bars plot average monthly S&P 500 returns.
The green bars show the monthly average from the 1950s.
The red bars show the past 5-year average.
The gray bars show the past 10-year average.
Daily September Seasonality: The lines plot the S&P 500’s average daily path through September.
The green line shows the daily average from the 1950s.
The red line shows the past 5-year average.
The gray line shows the past 10-year average.
The Takeaway: September is the market’s soft spot.
The only month where the S&P 500 averages a loss across decades of data. And in recent years, the damage has been worse, with the 5- and 10-year records sinking below the long-term baseline since the 1950s.
But the tape doesn’t always follow the calendar.
In bull markets, seasonal tendencies often bend or break.
This year, the index already shrugged off August’s weak seasonal tendency.
Trend strength and breadth kept the S&P 500 grinding higher even when history pointed lower.
If September delivers its usual downwards chop, it could reset positioning before the strong Q4 window. But if the market resists again, it will be another clear sign that this bull is overpowering seasonality.
Either way, month 9 is when history says trouble shows up.
So which wins this time — the seasonal tendency, or the trend?
Grant Hawkridge | Chief Aussie Operator, All Star Charts
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