Triple witching hour:
The final hour of the stock market trading session on the third Friday of March, June, September, and December, when option contracts and futures contracts expire on market indexes used by program traders.
The simultaneous expirations often set off heavy trading of options, futures and the underlying stocks.
Double Witching Hour:
The final hour of the stock market trading session on the third Friday of all months other than March, June, September, and December, when stock option and index option contracts expire.