Triple Witching and Expiration-Day Effects

"Triple witching" refers to the third Friday of March, June, September, and December, when stock index futures, stock index options, and single-stock options all expire simultaneously. Traders who used options or futures to hedge a position, or funds tracking an index that has just rebalanced, need to close out or roll expiring contracts, and much of that activity is deliberately timed into the closing auction because that is the price used to settle the expiring contracts. The result is a spike in trading volume in the final minutes of the session — often several times a normal day's closing-auction volume — concentrated in index constituents and heavily optioned names.

This mechanical, price-insensitive order flow can push prices briefly away from where fundamentals would put them, since market makers unwinding hedged option positions (delta-hedging into expiration) generate large, correlated buy or sell pressure that has nothing to do with news. The effect is typically short-lived: prices tend to revert in the days following expiration once the artificial rebalancing flow has cleared, which is why some strategies specifically watch for temporary mispricings around the close on witching days.