The last day on which you can exercise an option contract and buy or sell the stock at the strike price. After this date, the option becomes worthless and you lose any money you paid for it.
You buy a call option with an expiration date of March 15th. You have until the end of the trading day on March 15th to decide whether to exercise your right to buy the stock. If you don't exercise by that date, the option automatically expires and you cannot use it anymore.
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