When the buyer of a call option exercises the option. In options trading, the buyer of a call option can exercise his or her right to purchase or sell the underlying asset (such as a stock) at the exercise price or strike price.

Buyers of options can either exercise their right to buy the underlying security or they can let the option expire wothless. A call over can take place throughout the life of the option until the exercise cut-off time that falls on the last trading day prior to the option contract's expiration.