Triple witching hour is the last hour of the stock market trading session (3:00-4:00 P.M., New York Time) on the third Friday of every March, June, September, and December. Those days are the expiration of three kinds of securities:

* Stock index futures.
* Stock market index options.
* Stock options.

The simultaneous expirations generally increases the trading volume of options, futures and the underlying stocks, and occasionally increases volatility of prices of related securities.

A little history abt it :

The term “triple witching” is conventionally thought to originate from the three witches in Shakespeare’s play Macbeth. While the terms “double witching” and “quadruple witching” are sometimes used too, it doesn’t carry the same foreboding connotation as triple witching. The phrase is intended to connote the extra volatility leading up to the event resulting from the expiration dates of three financing instruments.