Are Mondays bad for stocks?

What do people believe about Mondays?

The belief: Mondays are bad days for stocks. Bad news piles up over the weekend, the story goes, and gloomy investors sell at the start of the week.

It's also called the weekend effect. Each January it gets a fresh coat of paint, when a date dubbed Blue Monday, the gloomiest of the year, does the rounds.

Blue Monday is a label for a feeling, not a market event, and we didn't test that one date. In 2027 it falls on Martin Luther King Jr. Day, when the market is shut anyway. Stocks get the day off from feeling blue.

What exactly did we test?

We compared the market's return on Mondays with its return on Tuesdays to Fridays.

That gave us 4,838 Mondays from July 1926 to August 2026, against 19,933 ordinary days (Tuesdays to Fridays).

The 388 first trading days after a Monday holiday, such as the Tuesday after Labor Day, sit in neither group. They follow a long weekend but aren't Mondays.

We measure every return above what cash in Treasury bills earned on the same day. When we compare two averages, the gap is in percentage points, or points for short. A minus sign means Mondays did worse.

What did we find?

Mondays really have been the weak day, by a wide gap over the century and a slim one in the last 20 years.

After subtracting what cash earned, the average Monday came out at -0.10%, against +0.06% for an ordinary day. That's a gap of 0.16 points a day.

Mondays also beat cash less often: 48% of the time, against 55% of ordinary days.

The gap's likely range is -0.19 to -0.12 points, well clear of zero. It showed up in the early half of our history (-0.26 points) and the later half (-0.06 points).

In the last 20 years the gap was -0.04 points, on 944 Mondays and 3,985 ordinary days. Mondays averaged +0.02% above cash, against +0.06% for ordinary days.

Mondays even beat cash 53% of the time lately, against 55% for ordinary days. So Mondays still trail. But Mondays losing money? Not lately.

Is Monday the worst day of the week?

Over the century, yes, and only just in the last 20 years: Monday was the lowest weekday both times.

After subtracting cash, Mondays averaged -0.10% over the century, while Tuesdays averaged +0.04% and Fridays +0.06%.

In the last 20 years Mondays averaged +0.02%, about level with Thursdays and well behind Tuesdays at +0.11%. Fridays averaged +0.04%. Daily returns swing a lot, so even the gap between Mondays and Tuesdays could be luck.

The first trading day after a Monday holiday was weak, too. It averaged -0.09% over the century and -0.11% in the last 20 years, though on only 100 days.

Did it fade once it became famous?

Not by our verdict rule, but it shrank a lot after 1980, so much that the gap since then could be luck.

Academic studies in 1980 and 1981 made it famous. Before 1980 the gap was -0.24 points; since then it has been -0.06 points. In the last 20 years it was -0.04 points.

The 1980s were still weak; the shrinking shows from the 1990s on. In the 1990s and the 2020s so far, Mondays actually did a little better than ordinary days.

The likely range since 1980 is -0.12 to -0.01 points, below zero, but only just. We widened the range because we checked several things at once; then it is -0.14 to +0.02 points, which just includes zero. So since 1980, the gap could be luck.

One common story is that once a pattern gets famous, traders may act on it and shrink it.

Does it hold everywhere?

Yes: it shows up across industries and in five-day weeks alone.

Over the century, Mondays trailed ordinary days in all 43 industries with enough history. That is 43 of the 49 US industry portfolios we checked.

Industries move together, so these aren't independent votes. Still, the effect isn't one odd corner.

One quirk of old data: before June 1952 the market also traded on most Saturdays, so many Mondays followed a Saturday session. We kept those years in. Counting only five-day weeks, from 1952, the gap was -0.12 points, and -0.04 points in the last 20 years.

Why might it happen?

Nobody knows for sure, and we didn't test the reasons. The usual guesses are bad news released over the weekend and investors in a sour mood on Monday morning.

Whatever the cause, the gap has mostly shrunk since it became famous. The Monday blues are now more of a mild Monday grey.

What does this mean for you?

MATH doesn't mean you should trade it. The gap is a fraction of a percent a day.

Take an example: step out of stocks into cash for every Monday, and hold stocks the rest of the week.

Over the century it edged ahead: +10.3% a year in total return, against +9.9% for staying invested. That's before taxes, at a twentieth of a percent per switch.

Its worst fall was -62.9%, against -83.9%. In the last 20 years it was -42.6%, against -54.7%. Part of that is just sitting in cash one day in five.

But it switches about twice a week, 92.6 times a year (94.6 lately). At a quarter of a percent per switch, its total return falls to -8.4% a year, and -12.3% in the last 20 years.

In the last 20 years it lagged badly even at the small cost: +6.0% a year in total return, against +11.5% for staying invested. With no trading costs at all, its total return was still behind, at +11.1%.

So this is not a recommendation. The opposite bet, holding stocks only on Mondays, would be its own untested idea.

What else did we test?

See all the calendar effects we tested, or how we test a myth.