Triple Witching

No, this one doesn't describe that scene from Macbeth.

Triple witching has to do with the simultaneous expiration of various derivative contracts. Four times a year, stock index futures, stock index options, and options for individual stocks all expire on the same day. It happens on the third Friday of March, June, September, and December (so, once a quarter, if you're thinking in corporate time).

These expirations can cause some fluctuations in the stock market, as options and futures get exercised, triggering some transactions that don't have anything to do with whatever else is happening on that Friday. It's like hitting turbulence in a plane. Things get bumpy for a bit (in this case, especially in the last hour of the trading session), but it doesn't impact long-term trends.