Reddit Inc. will be added to the S&P 500 before trading begins on August 18, replacing AvalonBay Communities Inc., after S&P 500 constituent Equity Residential agreed to acquire AvalonBay Communities. S&P Dow Jones Indices said the combined company is expected to be renamed Vivmark Residential and remain in the S&P 500 once the deal closes, subject to final conditions. In a separate change, Sun Communities Inc. will join the S&P MidCap 400 before the open on August 20, replacing Webster Financial Corp., which is being acquired by Banco Santander S.A. Index reshuffles like these are typically tied to mergers and are closely watched because funds tracking the benchmarks often need to rebalance holdings around the effective dates.