In trading, the term good for day refers to an instruction which can be attached to a broker order. Adding this instruction to an order turns that order into a day order. A day order expires if it cannot be fulfilled within the same trading day.
Example: You want to buy shares in a stock just ahead of its ex-dividend date in order to take advantage of the issuing company’s generous dividends. The day before the ex-dividend date you place a buy order for the stock with your broker. Because you are only interested in buying the stock in order to receive dividends, you add a good for day instruction to the order. If the broker cannot buy the shares you want by the end of the trading day, they cancel the order and do not buy the shares at all.