In the most abstract terms, it is simply a "Mega Volatility Day." Institutional players are forced to trade billions of dollars at the exact same time, causing wild and unpredictable price swings.
Quadruple Witching Day occurs four times a year, on the third Friday of March, June, September, and December. It is the specific day when four different types of financial derivative contracts expire simultaneously.
The four contracts that expire are:
Market Impact: Because large institutional investors must close out or roll over their massive positions before the deadline, the stock market experiences a huge surge in trading volume. This often leads to erratic and highly unpredictable price volatility, especially during the final hour of trading (known as the "witching hour").