What is the Monday effect in the stock market?

It is a day-of-week pattern where average stock returns on Monday were historically negative while other days, especially Friday, were positive. Kenneth French documented it in a 1980 Journal of Financial Economics study, and because Monday's return spans the weekend it is also called the weekend effect.

Does the day-of-week effect still work today?

Largely no — it is a classic example of an anomaly that faded, disappeared, or reversed in many markets after it became well known. The lasting value is the method: splitting a strategy's returns by day of the week and testing on recent data rather than trusting the old headline.

How do I test a day-of-week effect on futures or forex?

Split your strategy's results by weekday and look for a day whose average return clearly stands apart, then confirm it holds on recent data with a walk-forward test. In TapeScript you can view average return by day of the week for index futures, gold, or forex pairs in one step.

The Monday Effect: Does the Day of the Week Really Move Markets?

Here is a question that sounds silly until you look at the data: does it matter which day of the week you trade? For decades, careful research said yes. Returns were not spread evenly across Monday through Friday — and Monday, in particular, was strangely weak. It became one of the first "anomalies" that made academics admit markets were not perfectly random.

Who found it, and where (the who / what / when)

The classic paper is "Stock Returns and the Weekend Effect" by Kenneth French, published in the Journal of Financial Economics in 1980. French is a giant of finance research — the same French of the famous "Fama-French" factor models. A companion study by Gibbons and Hess in 1981 confirmed the pattern. This was not a fringe claim; it came from the top of the field.

What they actually did (the how they studied it)

French took decades of daily stock-index returns and simply sorted them by the day of the week. Then he compared the average return for Mondays, Tuesdays, Wednesdays, Thursdays, and Fridays. The test was almost embarrassingly simple — and that simplicity is why it was so convincing.

What they found (the data points)

The average return on Monday was negative, while the other days — especially Friday — were positive. Because Monday's return actually covers the move from Friday's close through Monday's close, people call it the "weekend effect." The gap was statistically real, not a rounding quirk. Later research found the effect weakened over time and even flipped in some markets and periods — an important lesson we will come back to.

The "equation" is just a label

There is no formula, only a way of grouping days. Each trading day gets a label:

Average return for each weekday = mean of all daily returns that landed on that weekday

So you take every Monday return over your data and average them, then do the same for Tuesday, and so on. If one weekday's average sits clearly below or above the others — and stays there across many years — you may have a day-of-week effect. That is the entire idea.

Why it should work (the why)

Several honest theories, none certain. One is settlement and cash-flow timing — historically it took days to settle trades, which shifted the cost of holding over a weekend. Another is information flow: bad news tends to be released after Friday's close, so it gets priced in on Monday. A third is mood — the "Monday blues" showing up in trader behavior. The truth is probably a mix, and because the causes are structural and behavioral, they can change as markets modernize.

Does it still hold — honestly?

This is the most important section for this one. The day-of-week effect is the poster child for anomaly decay: it was strong in old data, but it faded, disappeared, or reversed in many markets after it became famous. So do not trade the 1980 result on faith. What is still valuable is the method: splitting your own strategy's results by day of the week to see if an edge is lopsided. On futures and forex, day-of-week patterns can still show up — for example, currency pairs behaving differently early in the week — but you must prove it on recent data, not borrow a 40-year-old headline.

Build and test it in TapeScript, step by step

Create a testable version. Type: "Build a strategy on daily ES that goes long at the Friday close and exits at the Monday close, and separately one that is long only on the day after the weakest weekday."
Classify and baseline. Type: "Classify this and run the baseline backtest over all available years."
See the day-of-week map. Type: "Open the Time and Context view and show me average return by day of the week." This is the exact chart French built — you will see instantly whether one day stands out.
Test for decay. Type: "Run a walk-forward test by decade — is the day-of-week edge still present in the last 10 years, or has it faded?" This is the whole ballgame for this pattern.
Check other markets. Type: "Show the day-of-week return map for Nasdaq futures, gold, and EURUSD — does any day stand out today?"
Be honest with the holdout. Type: "Run Monte-Carlo and spend the untouched holdout — is what is left real or noise?"

The bottom line

The Monday effect is a real piece of finance history — and a real warning. It shows that markets have patterns, but also that patterns fade once everyone knows them. The lesson is not "sell on Mondays"; it is "always split your edge by time and check it on recent data." TapeScript makes that a one-line request and tells you honestly whether a day-of-week edge still breathes. Test your strategy by day of week →

Citation: French, K. R. (1980). "Stock Returns and the Weekend Effect." Journal of Financial Economics, 8(1), 55–69. See also Gibbons, M. & Hess, P. (1981), Journal of Business. Free versions on Google Scholar.