SEBI is rolling out the Closing Auction Session (CAS) from August 3, 2026, replacing the old VWAP-based closing price method for F&O-eligible stocks like Reliance, HDFC Bank, Infosys, and TCS. This guide breaks down exactly how CAS works - the minute-by-minute trading timeline, which stocks are covered, why stop-loss orders get cancelled during the auction window, and what changes for futures and options traders. We also explain how this new closing price mechanism indirectly affects mutual fund NAVs and index fund tracking, so both active traders and long-term investors know precisely what to expect before the rollout begins.