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What does Triple Witching mean?

An event that occurs when the contract for stock index futures, stock index options and stock options all expire on the same day.  Triple witching  days happen four times a year on the third Friday of March, June, September, and December.  When this phenomenon occurs each quarter, it gives the market a wild gyration, so much so that traders are so afraid to leave their post for fear that something drastic may happen to their positions, hence the term "Triple Witching" (something caused by witches)

Having said all that, these days, another expiration is also occuring that same day: the expiration of Single Stock Futures SSFs.  We now refer to that day as "Quadruple Witching".

never heard of it