Quadruple witching day

Once every quarter -- on the third Friday of March, June, September, and December -- stock options, stock index options, stock index futures contracts, and single stock futures expire on the same day in the United States.

In the past, when all contracts expired at the same hour of the day, trading could be extremely volatile as professional investors attempted to capitalize on pricing differences.

But in recent years, various adjustments in the trading schedule have helped to reduce the pace.