Check out our entire Terrifying Terminology series: Psychometrics, Zombie Institutions, Gross Domestic Product and Ghost People.

Illustration by Trent Troop

Four times a year, an unholy occurrence casts a spell over the stock market, stirring up mayhem and mischief most foul. On the third Friday of March, June, September and December the contracts for index futures, index options and regular stock options all expire. All that activity of closing out contracts tends to spook investors, says Jim Nolen, distinguished senior lecturer of finance.

Those days—known as triple witching—threaten to increase volatility in the market. For instance, as the contracts expire on index funds (bundles of different stocks meant to be representative of the overall market), fund managers may close out their positions or adjust their portfolios.

The volume of trading typically takes a particularly sharp increase during the last hour of the trading day—by as much as 5-10 percent. It’s a small percentage, but still significant, as that may represent millions of shares, says Nolen.

The activity also could trigger movement among program traders—automated trading based on algorithms that call for buying or selling once stocks hit a certain price. (For an example of how quickly volatility and program trading can spiral out of control, one need only look at the fallout of this May’s “fat finger” incident, when a trader allegedly typed “billion” instead of “million,” setting off a wild day of trading that resulted in the largest single-day drop in the history of the Dow Jones industrial average.)

So is anyone safe from the dark forces of triple witching? While day traders may live and die by the market’s daily gains and drops, most long-term investors will survive with little more harm than a wart or two.