Expiration is the deadline built into every option. At expiry, an option is either in the money (it has intrinsic value and settles for that amount) or out of the money (it expires worthless). There is no tomorrow for that contract - which is the entire point of the clock.

Exercise and assignment

Exercise is the option holder using their right to buy or sell at the strike. Assignment is the flip side - the seller of that option being obligated to fulfill it. Most retail traders close their options before expiration rather than exercise, but in-the-money options are typically auto-exercised at expiry, which can leave an unwary trader with an unexpected stock position.

Why OpEx moves markets

Big expiration events - monthly and quarterly "OpEx" - can move the whole market because enormous open interest expires at once. As those positions settle and the dealers hedging them unwind, the flows can amplify or dampen price around key strikes, contributing to effects like pinning. Daily-expiring products like SPY spread a version of this across every session.

Expiration is not just a date on a contract. On big OpEx days, it is a force acting on the whole tape.

The trader's takeaway

Know your expiration before you enter, and respect what it does to time decay as it approaches. The closer to expiry, the faster extrinsic value bleeds and the more violently price can whip around big strikes. Short-dated trading is a deliberate choice to operate in that high-speed zone - which rewards a clean plan and punishes drift.