Double Witching: translation
Similar to triple witching, but instead of three classes of options or futures expiring on the same day, double witching is when only two classes (any two) are expiring. The three classes are stock options, index options, and index futures.
In other words, this is when option contracts and futures contracts expire on the exact same day.
Double and triple witching days can be volatile at times as arbitrageurs scramble to close out their positions.