For those who don't know:

An event that occurs when the contracts 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.

This phenomenon is sometimes referred to as "freaky Friday".

Investopedia Says... The final trading hour for that Friday is the hour known as triple witching. The markets are quite volatile in this final hour, as traders quickly offset their option/futures orders before the closing bell. If you are a long-term investor, triple witching will have a minimal impact on you.