"It is not unusual to have higher volatility in December," HSBC's Chan said, noting that volume tends to trail off as fund managers close their books for the year, making it easier for large orders to sway the market either up or down.
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Indeed, since 1950, the S&P 500 has averaged a 1.7 percent gain in December, suggesting markets won't be sitting still.
End-of-year vacations can also put a damper on volume -- as well as spur analysts to publish before leave begins.
To an extent, leaving on vacation is a sign that market players feel confident there aren't any black swans flocking on the horizon.
Former Bear Stearns CEO James Cayne became a poster-child for ill-timed vacation planning. In 2007, amid a nascent global financial crisis, two of his company's hedge funds were melting down, faced with demands for redemptions and more collateral. At the time, Cayne was competing in a bridge tournament without access to a mobile phone or e-mail, a blunder that cratered the market's confidence in Bear Stearns, which collapsed in March of 2008 and was sold for $2 a share to JPMorgan.
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Of course, in the financial industry, vacation timing isn't always a choice. In the wake of Societe Generale's experience with rogue trader Jerome Kerviel, who racked up losses of around 5 billion euros by early 2008, many banks now require annual leave, sometimes proscribed at year-end, to allow others to examine trading books.
Kerviel eschewed the French love of vacations and didn't take time off, then rather famously declared: "A trader who doesn't take vacation is a trader who doesn't want to let anyone else look at his book."
But it may not just be the vacations pushing analysts to publish earlier. It may just be year-end "tis the season" optimism.
"Keep your fingers crossed and pencil in next year as a better year and then you get the revisions downward," Song said. "One year it will be all right."
—By CNBC.Com's Leslie Shaffer; Follow her on Twitter @LeslieShaffer1