SEBI has revised the Closing Auction Session (CAS) framework and derivatives settlement methodology, with two separate implementation dates. Most changes take effect on 7 December 2026. The provisions for carrying unexecuted limit and iceberg orders into CAS take effect on 8 February 2027. They should not be described as changes already in force today.
The regulator’s 15 October circular matters because a share’s cash-market closing price, the auction’s reference price and an expiring derivatives contract’s settlement price are not interchangeable. The circular sets out different calculations and trading cut-offs for these purposes. Times below are Indian market times (IST). The CAS provisions apply to securities covered by that framework, not an assertion that every listed security uses the auction.
| Check | Verified detail | Primary evidence |
|---|---|---|
| Official record | 15 October 2026 circular; HO/47/11/11(3)2025-MRD-POD2/I/23843/2026 | SEBI |
| Main implementation date | 7 December 2026, excluding paragraph 4.8 | SEBI |
| Order-transition implementation | 8 February 2027 for paragraph 4.8 | SEBI |
| CAS timing | 3:30–3:40 p.m.; order entry closes randomly from 3:38 to 3:40 p.m. | SEBI |
| CAS reference-price window | Last 15 minutes of continuous cash trading; specified fallbacks if no trades | SEBI |
| Expiry-day derivative settlement | Underlying cash-market last-30-minute VWAP methodology, with distinct index/stock provisions | SEBI |
| Derivatives trading cut-offs | Generally 3:45 p.m.; expiring index derivatives stop at 3:30 p.m. on expiry day; stock derivatives continue to 3:45 p.m. | SEBI |
| Cash post-close session | 3:50–4:00 p.m. at the closing price | SEBI |
Why 15 minutes and 30 minutes both appear
VWAP is an average price weighted by the quantity traded: sum of price × quantity divided by total quantity. CAS uses a last-15-minute cash-session reference price, subject to the circular’s fallback rules. Expiry-day index settlement instead uses an index value based on the constituent shares’ last-30-minute cash-session VWAPs. Stock derivatives use the underlying share’s last-30-minute cash-session VWAP across exchanges. A reference input to an auction is not automatically the auction’s discovered closing price.
A separate date for pending orders
Paragraph 4.8 deals with unexecuted limit and iceberg orders. An iceberg order reveals only part of its quantity during normal trading. The revised transition rule carries eligible pending iceberg orders into CAS as fully disclosed limit orders unless the participant opted out when placing the original order. Different time-priority conditions apply. This part starts in February, not with the main December changes.
What participants should verify before changing orders
Read the exchange and broker implementation notices for the relevant segment, order type and contract. A limit order specifies a price boundary; a market order does not provide that same price limit. The CAS order-entry stages have different restrictions, including no market-order modification or cancellation from 3:36 p.m. The circular is not a promise of execution, better prices or lower risk. This explainer does not claim that either future implementation milestone has already occurred.
Related coverage: MRPL’s separate record and payment dates. That corporate-action timetable is not the CAS implementation schedule.
Sources and reporting limits
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