Triple witching hour is the last hour of the stock market trading session (3:00-4:00 P.M., New York City local Time) on the third Friday of every March, June, September, and December. Those days are the expiration of three kinds of securities:
The simultaneous expirations generally increases the trading volume of options, futures, and their underlying stocks, occasionally increasing the volatility of prices of related securities.
On those same days single-stock futures also expire, so that the final hour is sometimes referred to as the quadruple witching hour.[1]