Sell in May / Summer Effect
SUMMER EFFECT
What is the summer effect in the stock market?
The “summer effect” is a broad label for weaker or less consistent stock-market behavior during parts of the May–October period. It overlaps with Sell in May, but should be tested by exact dates rather than treated as a fixed seasonal law.
Why summer seasonality is difficult to generalize
Different months inside the summer half can behave very differently. June may look nothing like September, and individual stocks can show patterns that are unrelated to the broad index.
Month-by-month differences
Breaking May–October into individual months can reveal where weakness is actually concentrated.
Market differences
U.S. and European indices do not always share the same seasonal path.
Stock-specific behavior
Company-level cycles can overwhelm broad market seasonality.
| Window | What it helps answer |
|---|---|
| May–October | Does the broad weak-half effect exist? |
| June–August | Is there a distinct summer pattern? |
| September only | Is one month driving most of the weakness? |
Test custom summer windows
Choose any date range and compare it across historical years in the free dashboard.