In one line
Expiry is the date the contract ends, and an Indian index option ends in cash: the exchange pays the difference between the strike and the settlement level, and everything else about the contract stops existing.
How it works
NIFTY carries weekly contracts and monthly ones; BANKNIFTY has been monthly only since NSE discontinued its weeklies in November 2024. That difference is not trivia — it changes what a chain looks like. A NIFTY board in the middle of a week holds a contract expiring in three days; a BANKNIFTY board on the same day may hold nothing nearer than three weeks, and the two price volatility very differently as a result.
The settlement level is not the last traded price. For an index it is computed from a window of prices near the close on expiry day, which stops a single print at 15:29 from deciding what every contract pays. The desk stores the settled level from the exchange's own files rather than from a quote.
Days to expiry is the number every part of this desk sorts on, and it is the reason the board never picks a contract by the name of its bucket. The bucket names are w0, w1, m0, m1 and m2 — and m0 sorts before w0 in the alphabet, so ordering by name once put the MONTHLY contract on every page of this site. The fact the name stands in for is days to expiry, so that is what is ordered on.
What it is not
Expiry day is not a separate market with its own rules. It is an ordinary session in which one particular contract has no time left, and the effects traders describe as expiry behaviour are mostly that: time value that was worth something on Monday is worth nothing by Thursday afternoon, whatever the index does.