The expiration date is the last day that an option contract or contracts (in the case of a spread) are valid. On or before this day investors should decide what to do with the expiring position, let it automatically exercise/assign if in the money, let it expire worthless if out of the money, or trade out of the position at its current mark to market price (for a profit or a loss from entry).
Options expire on Friday, with some ETFs like SPY having additional expirations such as after the close Monday or Wednesday. The trader should be aware of their exact expiration date for each position.
An option position’s time value will decrease towards zero as expiration approaches. Additionally, that position’s delta will rapidly approach either 100 (for in the money) or 0 (for out of the money) into expiration. Whether a stock is above or below the strike on expiration might mean the difference between 100 deltas (and a stock position) or 0 deltas (and no position). Therefore, monitoring expiring positions closely is important and most traders choose to close the position before it expires.