Reddit (RDDT) will replace AvalonBay Communities (AVB) in the S&P 500 effective August 18th

Index additions and deletions of this kind follow a well-worn mechanical sequence: passive funds benchmarked to the index must buy the entrant and sell the departing name at the effective date's closing print, which concentrates the flow into the rebalance-day close and typically produces elevated volume and a closing auction imbalance in both names. The pattern in past inclusions has been a run-up in the entrant between announcement and effectiveness as fast money front-runs the passive bid, with some tendency toward partial retracement after the print once the forced buying is done. The deleted name tends to see the mirror image, though the selling pressure there is often better absorbed given the stock's established institutional holder base. Worth noting is that additions of younger, high-beta names have historically carried an outsized weight of retail and momentum flow alongside the index bid, which can exaggerate both legs of the move relative to a routine reconstitution. The tells are the closing auction imbalance indications on the effective date and whether indexers execute early or at the print. As an index event rather than a fundamental one, the flow is predictable in direction even if the magnitude is not.