Claim check: The Monday effect
“Monday is the worst day of the week for stocks.”
What the data says
Does not hold Not supported by the data: Monday returned +0.06% a day on average against +0.06% for other days in S&P 500 (n = 793 days, 2010-01-04 to 2026-11-10; t = 0.2, likely noise; rank 3 of 5 by average return). We see no sign of the claimed effect.
No effect we can tell from chance, or the opposite effect.
Who says it
Documented by Frank Cross (1973) and French (1980) 'weekend effect'.
Origins are as commonly cited and were not independently verified.
How we tested it
Daily close-to-close returns of SPY grouped by weekday. Monday is compared with all other days (Welch t-stat), must have the lowest average of the weekdays and a negative difference in both halves of the history. A Monday return covers the weekend, so it is the previous Friday close to Monday close.
Results
| Market | Days | Mean/day | Other days | Difference | Up share | t-stat | Rank | Reading |
|---|---|---|---|---|---|---|---|---|
| S&P 500 | 793 | +0.06% | +0.06% | +0.008% | 57% | 0.2 (likely noise) | 3 | No effect |
Average daily return by weekday (S&P 500)
Stability across the two halves of history
A saying that only works in one half of the history is not much of a rule. Each row shows the effect in the full history, the first half and the second half; green means the claimed effect was present.
| Market | Full history | First half | Second half |
|---|---|---|---|
| S&P 500 | +0.01% | -0.03% (394) | +0.05% (399) |
Context and cross-checks
Shown for context; not part of the verdict.
Other markets (context, not part of the verdict)
| Market | Days | Mean/day | Other days | Difference | Up share | t-stat | Rank | Reading |
|---|---|---|---|---|---|---|---|---|
| Gold | 793 | +0.02% | +0.04% | -0.019% | 51% | -0.4 (likely noise) | 2 | Right way, likely noise |
| Silver | 793 | +0.10% | +0.03% | +0.069% | 51% | 0.9 (likely noise) | 5 | No effect |
| Commodities | 793 | -0.03% | +0.02% | -0.053% | 47% | -1.2 (likely noise) | 1 | Right way, likely noise |
| Agriculture | 793 | +0.01% | +0.01% | +0.002% | 47% | 0.1 (likely noise) | 3 | No effect |
| EUR/USD | 879 | -0.03% | 0.00% | -0.034% | 48% | -1.7 (likely noise) | 1 | Right way, likely noise |
| Bitcoin | 634 | +0.49% | +0.12% | +0.363% | 54% | 2.3 | 7 | No effect |
| Ether | 470 | +0.28% | +0.13% | +0.149% | 52% | 0.6 (likely noise) | 5 | No effect |
Average daily return by weekday (Gold)
The honest caveat
Close-to-close Monday return mixes weekend news with Monday trading, so cause is unclear.
Descriptive statistics from historical data, before costs. Not a forecast and not investment advice.
Questions
Monday is the worst day of the week for stocks.
Not supported by the data: Monday returned +0.06% a day on average against +0.06% for other days in S&P 500 (n = 793 days, 2010-01-04 to 2026-11-10; t = 0.2, likely noise; rank 3 of 5 by average return). We see no sign of the claimed effect.
How was this claim tested?
Daily close-to-close returns of SPY grouped by weekday. Monday is compared with all other days (Welch t-stat), must have the lowest average of the weekdays and a negative difference in both halves of the history. A Monday return covers the weekend, so it is the previous Friday close to Monday close.
Markets used
S&P 500 · Gold · Silver · Commodities · Agriculture · EUR/USD · Bitcoin · Ether