Quadruple Witching

A day on which contracts for stock index futures, stock index options, stock options and single stock futures (SSF) all expire.

Investopedia Says:
This is similar to the triple witching hour, except that the quadruple witching hour sees also the expiry of SSFs.

Quadruple witching days occur on the third Friday of March, June, September and December.

Related Links:
An introduction to the world of options, covering everything from primary concepts to how options work and why you might use them. Options Basics Tutorial
For those who are new to futures but want a solid understanding of them, this tutorial explains what futures contracts are, how they work and why investors use them. Futures Fundamentals