Index funds could experience slightly more trading than usual on Friday. Company’s shares outstanding change from time to time due to buy-backs, issuance for employee options or other events. The percentage of shares in float — not closely held — also change from time to time. In theory S&P Dow Jones could update data on the number of shares outstanding for companies in its indices daily; but most of these changes are small and dealing with numerous tiny changes would be costly for index fund managers. As a result, changes of less than 5% of the number of shares outstanding are done once a quarter on the third Friday of the last month of the quarter — next Friday, June 21st. This is the same day that futures and options contracts expire, so there is plenty of liquidity in the market. These are all small adjustments since large share changes related to mergers are done when the merger closes. Aside from slightly more trading than typical for Friday in summer, there isn’t likely to be any impact. The indices affected include the S&P 500, S&P MidCap 400 and the S&P Small Cap 600, among others. The Dow Jones Industrial Average is price-weighted: stock prices, but not share outstanding, matter for the calculation so there is no rebalance for the Dow.