Monday is the only day the stock market is more likely to fall than to rise. The Dow Jones industrial average has been down 10 of the past 11 Mondays. And the two worst days in market history are both known as Black Monday.
There's no single reason why Mondays are so blue. Then again, there's no single reason the market rises or falls on any given day, driven as it is by the whims of traders placing millions of individual buy and sell orders.
Some anecdotal evidence comes to mind: Companies are prone to release bad news on Friday nights, when fewer people are paying attention. Monday is the first day investors can react.
And when companies collapse, they often do it late Sunday or early Monday, after spending a last weekend trying to stay afloat. See Wachovia, Bear Stearns and, most famously, Lehman Brothers investment bank, on Sept. 15, 2008.
Maybe people are just grumpier. They are at least more anxious: The so-called Vix, a gauge of investor fear, tends to go up on Mondays, notes Ryan Detrick, senior technical strategist for Schaeffer's Investment Research in Cincinnati.
The Vix has risen on two-thirds of this year's Mondays. On Tuesdays, the second-most-anxious day, the Vix was up just 58 percent of the time.
Or maybe it's a fluke - another pattern people latch on to to make the market seem more understandable, same as the stories that hemlines go up in bull markets, or that stocks rise if a team from the NFC wins the Super Bowl.
Burton Malkiel wrote about those last two theories in his finance classic, "A Random Walk Down Wall Street." He stuck them in a section called "A Gaggle of Other Technical Theories to Help You Lose Money."
He found the "blue Monday" phenomenon equally underwhelming. "Far from dependable," he says, and "most likely due to chance."
Still, there is a pattern.
Howard Silverblatt, senior index analyst for S&P Dow Jones Indices, crunched numbers for the Standard & Poor's 500 stock index back to 1928 and found that melancholy Mondays are a long tradition.
Over the past 84 years, the S&P has declined on 52 percent of the Mondays, Silverblatt says. Same goes for the Dow, going back to 1900. On each of the other four days, the market is more likely to rise than fall.
The S&P averages a decline of 0.12 percent on Mondays over history. On each of the other four days, the market averages a gain. (The best is Wednesday, averaging an increase of 0.08 percent.)
This year follows the pattern: For both the S&P and the Dow, Monday is the only day to average a loss.
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, 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," Duncan says. "Much like how the stock market works on Mondays."