triple witching hour

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The hour before the closing of the stock market on the third Friday of March, June, September, or December, when stock index options, stock index futures, and stock options expire, known for its high trading volume and volatile prices.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The final hour of trading during which three types of derivatives contracts expire.