It falls on the third Friday of March, June, September and December. Index futures roll or settle, index options settle, equity options expire, and index rebalances often print at the same close. The result is one of the highest-volume sessions of the quarter.
The move itself is usually noise: it is mechanical flow, not information. But liquidity is unusual, dealer-gamma flips as huge positions vanish, and the following Monday often behaves very differently once that positioning is gone.
Example: a typical session in an index ETF trades 70 million shares. On a triple-witching Friday it trades 250 million, with 60 million in the closing auction alone. A market-order sized normally still fills fine; a limit left near the close may fill at a price that never appears on your chart.
Related: opex, am-settlement, dealer-gamma, standard-expiration