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Stock Market · Options Theory

Expiry day mechanics

Since September 2025, expiry days in India are split by exchange: NSE index options expire on Tuesday, with monthly expiry on the last Tuesday; BSE Sensex options expire on Thursday. (Both used to be Thursday. Older material still says so.) One catch that trips people up: only NIFTY has weekly options on the NSE. Bank Nifty weeklies were withdrawn in November 2024, so it now has monthly and quarterly contracts only. Understanding expiry day behaviour is essential.

What happens on expiry day

• Expiring contracts trade until 3:40 PM. From 7 December 2026, expiring index options stop at 3:30 PM and expiring stock options at 3:45 PM
• Final settlement price for index options: index values from the closing auction. From 7 December 2026, index values built from each stock's average price between 3:00 and 3:30 PM
• OTM options expire worthless. ITM options are automatically exercised/cash-settled.
• Theta decay is at its maximum. Time value collapses to zero by expiry

The morning-of risk

If you're holding short OTM options that expire 'safely' OTM, a gap move at 9:15 AM or a sharp intraday move can put them ITM suddenly. What was worthless yesterday is now a loss.

Short sellers who 'let it expire' without monitoring expiry day can wake up to large losses if overnight news creates a gap.

The 'pin risk' phenomenon

As expiry approaches, large open interest at specific strikes can act as a magnet. Market makers and institutions with positions near that strike will actively trade to push/keep price near certain levels. Protecting their P&L. This is called 'pinning to a strike.'

Weekly vs monthly expiry behaviour

Weekly expiry: shorter life means higher gamma, faster theta decay, more explosive moves in ATM options near open.

Monthly expiry: more open interest, larger market participation, slightly more predictable settlement as major players roll or close.

3:00–3:30 PMSettlement window for NSE index options from 7 December 2026. Prices in this half-hour determine your final P&L.

Rule for retail traders

Close all option positions before 2:00 PM on expiry day if you're not specifically expiry-day trading. Don't be exposed to 30-minute settlement risk unnecessarily.

Takeaway. Expiry day options settle on the closing auction, and from 7 December 2026 on the 3:00–3:30 PM average. Theta collapses fully by close. OTM options expire worthless. Close positions before 2 PM unless deliberately trading expiry.