Stock Market · Options Theory

Expiry day mechanics

Since September 2025, expiry days in India are split by exchange: NSE index options (Nifty, Bank Nifty) expire every Tuesday, with monthly expiry on the last Tuesday; BSE Sensex options expire every Thursday. (Both used to be Thursday. Older material still says so.) Understanding expiry day behaviour is essential.

What happens on expiry day

• All options expire at 3:30 PM
• Final settlement price for index options: average of Nifty 50 values from 3:00–3:30 PM (every 15 seconds, ~120 data points averaged)
• OTM options expire worthless. ITM options are automatically exercised/cash-settled.
• Theta decay is at its maximum. Time value collapses to zero by 3:30 PM

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. 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 at 3:30 PM based on 3:00–3:30 PM average. Theta collapses fully by close. OTM options expire worthless. Close positions before 2 PM unless deliberately trading expiry.