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Notably horrible was Monday, June 11, when the Dow fell 142 points because of worries about Spanish debt. Almost as bad were June 25, also capsized by worries about Spain, and April 9, after an anemic jobs report.

“Maybe over the weekend, that’s when reality sets in,” says Tim McCandless, senior stock analyst at Bel Air Investment Advisors in Los Angeles.

Three of the five worst days in the history of the S&P 500 were Mondays, including two days known as Black Monday: Oct. 19, 1987, when stocks plunged more than 20 percent, and Oct. 28, 1929, which helped set off the Great Depression.

So pity the poor Monday. Even pop culture is stacked against it. The Mamas & the Papas sang that every other day of the week is fine. Nobody names a restaurant T.G.I. Monday’s. The Titanic sank on a Monday, for crying out loud.

It wasn’t always like this, with Mondays representing the dreaded beginning of the workweek in Western countries. Monday probably got its bad name when the Roman emperor Constantine invented the weekend, as David Ewing Duncan, author of a book on the history of the calendar, is fond of saying.

Constantine made Sunday a rest day, an attempt to please both sun-worshippers, who were already observing it, Mr. Duncan says, and Christians, who Constantine knew could be persuaded because they believed in the resurrection of Christ on a Sunday.

So Monday, named for the moon, became the day for going back to work.

Which, in some ways, is only fitting.

“There’s always been some mystery around the moon,” Mr. Duncan says. “Much like how the stock market works on Mondays.”