Quadruple Witching

Quadruple Witching refers to the 3rd Friday of every quarter i.e March, June, September and December.

This is the trading day when:

• market index futures, 
• market index options, 
• stock options and 
• stock futures

expire.

There'll be increased volatility on this day.

The last hour if these trading days (3-4pm EST) is referred to as Quadruple Witching Hour.

Ref: https://investinganswers.com/dictionary/q/quadruple-witching