Understanding the Weekend Effect in Stock Markets

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The weekend effect is a calendar anomaly in which stock returns on Mondays tend to be lower than returns on the preceding Friday. Because U.S. stock exchanges close on Saturday and Sunday, a Monday return covers the full stretch from Friday’s close to Monday’s close, including two non-trading days. Frank Cross documented the pattern in 1973 in the Financial Analysts Journal, finding that average Friday returns exceeded average Monday returns.

How the weekend effect is measured

Every daily return compares one closing price with the previous closing price. For most weekdays that gap is one trading session and one overnight period. The Monday return is different:

Monday return = (Monday close − Friday close) ÷ Friday close

That single number absorbs everything from Friday at 4 p.m. Eastern to Monday’s close: weekend news, economic or political developments overseas, and the Monday session itself. Because regular stock trading stops over the weekend, the accumulated information shows up all at once when the market opens, often as a gap between Friday’s close and Monday’s opening price.

To test for a weekend effect, researchers group daily returns by weekday over a long sample, often for a broad index such as the S&P 500 or the Dow Jones Industrial Average, then compare the average Monday return with the average Friday return. A persistent, statistically meaningful gap in which Mondays come out lower is the weekend effect.

Worked example (hypothetical)

Suppose an index closes at 5,000 on Thursday, 5,040 on Friday and 5,010 on Monday.

• Friday return: (5,040 − 5,000) ÷ 5,000 = +0.80%
• Monday return: (5,010 − 5,040) ÷ 5,040 = −0.60%

One week like this proves nothing. The weekend effect is a claim about averages across hundreds or thousands of weeks, and individual weeks can and do move the opposite way.

Why the pattern might occur

No single explanation has settled the debate. The main candidates:

Timing of corporate news. One theory holds that companies tend to release unfavorable announcements after the market closes on Friday, hoping they draw less attention. Investors then react when trading resumes, pushing prices down on Monday.

Individual investor behavior. Research points to individual investors as more active sellers on Mondays, particularly after bad news. People who review their portfolios over the weekend may arrive on Monday ready to cut positions.

Short selling. Another line of argument links the effect to short sellers, which would make it stronger in stocks with high short interest. Short positions carry risk over a weekend when no trading is possible, and the way traders open and close them around Friday and Monday may shape prices on those days.

Mood and optimism. A simpler explanation is that traders feel more upbeat heading into a weekend and that the optimism fades by Monday.

The reverse weekend effect and size differences

Not all studies find weaker Mondays. Research on a reverse weekend effect finds Monday returns that are actually higher than returns on other days. Some work suggests there are multiple weekend effects depending on firm size: small companies show lower Monday returns while large companies show higher ones. It has also been proposed that the reverse effect appears only in U.S. stock markets.

Distinctions readers confuse

Monday effect. The terms are sometimes used interchangeably, but the Monday effect is often defined differently: it holds that Monday returns follow Friday’s direction. If the market rose on Friday, the trend should resume on Monday, and if it fell, the decline should continue. The weekend effect instead describes Mondays underperforming Fridays.

Day-of-the-week effect. This is the broader category of anomalies in which average returns differ by weekday. The weekend effect is one specific member of it.

Weekend price movement. A listed share price does not change between Friday’s close and Monday’s open; what changes is the information investors will act on. Index futures reopen Sunday evening U.S. time and can signal how Monday may start, but that is a forecast of the open, not a stock price change.

Limitations

The weekend effect is a historical average, not a trading signal you can rely on for any given week. Its strength has varied: a Federal Reserve study found negative weekend returns before 1987, a disappearance of that pattern from 1987 to 1998, and higher weekend volatility after 1998. Once an anomaly is widely published, traders may act on it, which can erode it. The average gap between Monday and Friday returns is also small compared with ordinary daily swings, so trading costs, taxes and a single bad news event can easily outweigh any edge from timing purchases around the weekend.

Researchers who search many calendar patterns in the same data also risk finding gaps that arise by chance, and the conflicting results on reverse effects and firm size show how much the pattern depends on how the sample is built.

FAQs

The Bottom Line

The weekend effect matters most to investors tempted to time purchases around Friday and Monday. The pattern is a historical average whose strength has shifted over decades, and the typical gap is small next to ordinary daily swings. Trading costs, taxes or one bad news event can easily wipe out any edge, so it works better as a research finding than as a trading rule.