Markets are generally open from 9:30 am to 4:00 pm (EST) Monday through Friday (excluding holidays). Orders can only be executed during these hours of operation, but it is possible to queue orders to be submitted when the market next opens. 

For example, if you submit a market buy order to purchase 5 shares of XYZ at 8:00 pm (EST) on a Wednesday, it will be placed when the markets open at 9:30 am (EST) Thursday. For a more detailed explanation on how market orders that are submitted after-hours are treated, read our article here.