Major US Stock Index Will Now Shuffle Holdings Twice a Year. Here's Why That Matters.

FTSE Russell is adding a December rebalance to its annual June overhaul, starting late 2026. When indexes rebalance, funds that track them must buy and sell stocks in lockstep—creating a wall of orders. December's timing is brutal: fewer traders, year-end tax moves, and window-dressing collide. Expect bigger price swings twice yearly.