Triple witching
Quarterly expiry of stock index futures, index options and single-stock options on the same day, the third Friday of March, June, September and December.
Triple witching is the nickname for the quarterly day when three types of derivatives expire together: stock index futures, stock index options and single-stock options. It falls on the third Friday of March, June, September and December. Because all three settle at once, the day often has unusually high volume.
On our site, triple witching is marked on the calendar. Volume and closing-auction activity are usually higher than on an ordinary Friday.
Traders care because the combined expiry and rebalancing flows can cause sharp moves near the close, and index rebalances often fall on the same day.
Worked example (illustrative numbers, not real data): a stock with normal daily volume of 3 million shares trades 9 million on triple witching day. The extra volume reflects expiry and rebalancing flows, not necessarily new views, and the price may revert in the next session.