Stock Seasonality / S&P 500
S&P 500 SEASONALITY
How seasonal is the S&P 500?
The S&P 500 is one of the most useful benchmarks for studying recurring market behavior because it offers a long history and broad exposure to US equities. Seasonal patterns can be tested by month, quarter or custom date window.
What to test on the S&P 500
| Window | Why researchers test it | What to compare |
|---|---|---|
| January | January Effect and early-year positioning | Average, median and win rate |
| May–October | Sell in May hypothesis | Against November–April |
| September | Historically weak-month reputation | Recent vs long-term samples |
| Late December | Santa Claus rally research | Short window vs full December |
Why lookback length matters
A seasonal pattern can look strong over 25 years but weak over the most recent decade, or vice versa. That does not automatically invalidate the pattern; it tells you the regime may have changed.
Long lookbacks
Useful for seeing whether a tendency has existed through multiple market cycles.
Recent lookbacks
Useful for checking whether the effect still appears relevant in current market structure.
Test S&P 500 seasonality yourself
The dashboard opens with ^GSPC so you can select any date range and inspect each historical year.