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 Bank Nifty weeklies were withdrawn in November 2024, so it now has monthly and quarterly contracts only.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

• 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.