As a selling wave hit stocks Friday, traders chatted up a market factoid that appeared to lay the blame for the decline squarely on month end portfolio maneuvering.
They noted that stocks ended the last six months with a down day, six out of six times. But, stocks then traded higher more often than not when the new month began.
The Dow, for instance, traded positive four out of the last six times on the first trading day of the month. The Dow Jones industrial average fell on every final trading day of the month since November.
The S&P 500 has declined on the last day of trade for the month since October and has risen half the time on the first trading day of the month.
The Nasdaq Composite also gained on the first trading day in half of the last six months. The index fell on every last day of the month since November.
New-month cheer can often last beyond the first day.
A 1.3 percent loss in the S&P 500 on Jan. 30 was followed by a 1.3 percent gain on Feb. 2, and a 1.44 percent gain on Feb. 3. The index's 1.01 percent fall on April 30 came before gains of 1.09 and 0.29 percent on the next two days.
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The major indices closed about half a percent lower on Friday as investors eyed soft economic data and concerns about Greece. The session was choppy, with the Dow falling more than 150 points before ending 115 points lower.
"There is some sort of a calendar effect going on," said Jack Ablin, chief investment officer at BMO Private Bank.
—CNBC's Robert Hum contributed to this report.