Quantum BiasMonday Effect: Is Monday the Worst Day for Stocks?
One of the oldest calendar anomalies in finance, and what happened to it once it got famous.
In this article
For decades, yes. Today, not reliably. In 1980, Kenneth French showed that S&P 500 returns from Friday's close to Monday's close were negative on average between 1953 and 1977, while every other weekday was positive. Later research found the effect weakened or vanished after it was published, and that a lot of calendar patterns look stronger in the data than they really are. If you trade futures, the more practical point is that the "weekend" now ends with the Sunday-evening Globex open.
| Question | Data | Answer |
|---|---|---|
| Are Mondays systematically bad for U.S. stocks? | French (1980), S&P 500 1953–1977; later reviews by Sullivan, Timmermann and White (2001) and Schwert (2003) | Historically yes; the effect weakened after publication and is not a reliable pattern today |
The original finding
In March 1980, Kenneth R. French published Stock returns and the weekend effect in the Journal of Financial Economics (vol. 8, no. 1). Working with daily S&P 500 returns from 1953 to 1977, he asked a simple question: does Monday's return, which covers three calendar days, act like three days of returns or like one trading day?
The answer caught everyone off guard. Monday's return wasn't three times bigger. It was negative on average, while the other four weekdays were positive. Over those 25 years, holding stocks only over weekends would have lost money.
That was a big deal because it went against the most basic model of prices. If returns pay investors for time or for risk, a three-day stretch should earn more, not less.
What happened next
The weekend effect became one of the best-known anomalies in finance. Researchers confirmed it in other periods and countries, came up with explanations, and practitioners started talking about "Blue Mondays".
Then it faded. In Anomalies and Market Efficiency, first circulated as an NBER working paper in October 2002 and published in 2003 in the Handbook of the Economics of Finance, G. William Schwert looked at what happened to famous anomalies after they were published. His conclusion: the size effect, the value effect, the weekend effect and the dividend-yield effect seemed to have weakened or disappeared after the papers that pointed them out. He also noted that practitioners started launching investment products based on some of them around the same time. That raises the possibility that some anomalies were more apparent than real, or got traded away once people knew about them.
A second line of research questioned the original evidence itself. In Dangers of data mining: The case of calendar effects in stock returns (Journal of Econometrics, 2001), Ryan Sullivan, Allan Timmermann and Halbert White pointed out that researchers had combed through a huge number of calendar rules: days of the week, months, holidays, turns of the month. Once you account for how many rules were tried, the best ones look a lot less impressive.
- 01Gap from Friday's settlement to the Sunday 18:00 ET open
Shows how much weekend news was priced in immediately
- 02Overnight high and low by 9:00 ET
Gives the first reference prices of the week
- 03Scheduled data and Fed speakers on Monday
Mondays are often light on data, which can leave the overnight move unchallenged
A visual summary of the article's table — not a performance chart or a new backtest.
Why the effect might have existed
There are several explanations out there, and none of them is settled.
- Bad news on Friday evenings. Companies may drop bad news after Friday's close, so it hits prices on Monday.
- When individual investors trade. Retail investors may tend to sell on Mondays after looking at their portfolios over the weekend, while institutions trade less early in the week.
- Settlement and measurement quirks. Some early studies were sensitive to how settlement days and index calculations worked.
- Data mining. Like we said above, part of the effect may just come from testing a lot of calendar rules and reporting the one that worked.
The fact that it faded after publication fits several of these at once: part of it may have been real and traded away, and part of it may never have been solid.
The weekend in a 23-hour market
If you trade index futures in 2026, "the weekend" isn't a gap between Friday's 16:00 ET close and Monday's 9:30 ET open anymore. ES and NQ reopen on CME Globex at 18:00 ET on Sunday, so by Monday's cash open, the weekend's news has already been traded for more than 15 hours.
That changes the useful question. Instead of "is Monday bad?", it's:
- How big is the Sunday-evening gap from Friday's settlement?
- Does the overnight session extend or reverse that gap before the cash open?
- How does the cash session treat the overnight move? Our overnight vs intraday article covers what research has found about the two halves of the day.
You can measure those questions every week. A weekday label, you can't.
Why calendar patterns are fragile
The weekend effect is a good lesson in how to read any calendar stat, including the ones we publish, like Sell in May:
- How many rules were tried? Five weekdays, twelve months, dozens of holidays and month-end windows: with that many candidates, some will look strong by pure luck.
- Did it survive publication? A pattern that disappears once it's known was either traded away or never real. Either way, it's not something to rely on.
- Is there a mechanism? Calendar effects backed by a believable flow, like month-end pension flows, deserve more weight than ones without. Our article on the turn of the month is an example.
- How big is it after costs? A difference of a few hundredths of a percent a day disappears fast once you pay commissions and spreads.
What this means if you trade ES or NQ
- Don't build a Monday bias on the calendar alone. The historical effect is too weak today to justify a directional view.
- Do prep for the Sunday open. Weekend news gets priced from 18:00 ET on Sunday, so note the Sunday-evening open, the overnight high and low, and the gap from Friday before Monday's session.
- Track your own Mondays. If your results on Mondays look different from other days, the cause might be your process, like lighter prep after a weekend, not the market.
A short Monday checklist
Instead of a weekday rule, three checks before Monday's open do more for you:
| Check | Why it matters |
|---|---|
| Gap from Friday's settlement to the Sunday 18:00 ET open | Shows how much weekend news was priced in immediately |
| Overnight high and low by 9:00 ET | Gives the first reference prices of the week |
| Scheduled data and Fed speakers on Monday | Mondays are often light on data, which can leave the overnight move unchallenged |
None of these depends on it being Monday. They apply to any session after a long break, including the day after a holiday.
How to test it yourself
- Export daily closes for the market you trade, ideally ten years or more.
- Compute each day's close-to-close return and label it by weekday.
- Compare the average return and the share of up days for each weekday.
- Split the sample in two halves. A real effect should show up in both. A fluke usually shows up in only one.
- Shuffle the weekday labels many times to see how often a difference that big shows up by luck.
Limits
- We didn't run our own daily test. This article sums up published research. Our free data source doesn't provide long daily history.
- The original study covers 1953–1977, a very different market, long before electronic trading and 23-hour futures.
- Later evidence is mixed across countries and periods. We cite reviews rather than every single study.
Sources
- French, K. R. (1980). Stock returns and the weekend effect. Journal of Financial Economics, 8(1), 55–69. IDEAS/RePEc
- Sullivan, R., Timmermann, A., & White, H. (2001). Dangers of data mining: The case of calendar effects in stock returns. Journal of Econometrics, 105(1), 249–286. IDEAS/RePEc
- Schwert, G. W. (2003). Anomalies and market efficiency. In G. M. Constantinides, M. Harris & R. M. Stulz (Eds.), Handbook of the Economics of Finance (Vol. 1, ch. 15, pp. 939–974). Elsevier. Working paper version: NBER Working Paper 9277, October 2002. NBER
- CME Group. E-mini Nasdaq-100 Futures: Contract Specs (Globex hours). cmegroup.com. Accessed October 12, 2026.
FAQ
What is the weekend effect? The historical tendency for stock returns from Friday's close to Monday's close to be lower than on other days, and often negative.
Who discovered the weekend effect? Kenneth French documented it in 1980, using S&P 500 data from 1953 to 1977.
Does the weekend effect still exist? Reviews of anomalies found that it weakened or disappeared after it was published. It's not a reliable pattern to trade today.
What's the best day of the week to trade? Calendar data alone doesn't give a reliable answer. Liquidity and scheduled news matter more than the weekday.
When does the futures week start? ES and NQ reopen on CME Globex at 18:00 ET on Sunday, so weekend news gets traded long before Monday's cash open.
Research and education only. Not financial advice. Past results do not predict future returns.
How this article was made: drafted with AI assistance from the research papers listed above; facts and figures checked by Gabriel Décarie and Karel Panneton.
The week just closed, summarized, and the economic calendar for the week ahead.