Retail traders launched a 'No Trade Day' campaign on August 12 to protest SEBI's new Closing Auction Session (CAS) rule. CAS introduces a price discovery mechanism for equity market closing prices, replacing the earlier volume-weighted average price method. During CAS, traders place orders in a dedicated window and the final closing price is determined by auction equilibrium. Critics argue CAS increases volatility, disadvantages small traders, and creates artificial price swings in the final trading minutes.

The Closing Auction Session (CAS) is a market microstructure mechanism SEBI introduced to determine the official daily closing price of listed equity securities through a structured auction rather than a simple average. In a CAS, trading halts at a specified time, followed by a call auction window where buy and sell orders accumulate without matching. At the end of this window, a single equilibrium price is computed — the price at which maximum volume can be traded — and all eligible orders execute at that uniform price.

SEBI modelled CAS on global exchanges like the London Stock Exchange and NSE's own pre-open session design. The rationale is that a call auction at close reduces end-of-day price manipulation, smooths out last-minute order surges, and brings India's price-discovery mechanism in line with international best practices.

The controversy arises because CAS can concentrate volatility into a narrow window, making it harder for retail traders with slow internet connections or non-algorithmic setups to participate effectively. Institutional and algorithmic players, who can place and modify orders rapidly, tend to benefit disproportionately during auction windows.

For SEBI Grade A aspirants, the key exam angles are: the acronym expansion (CAS = Closing Auction Session), the purpose (closing price discovery), the mechanism (call auction equilibrium), and how it connects to SEBI's broader mandate of market integrity and investor protection under the SEBI Act, 1992.