For live market orders, buy and sell orders are only executed when the market is open, however you can place them at any time, except between 4:00 - 4:15 U.S. Eastern Time. 

Between 4:00pm - 4:15 ET, market orders are rejected and will not sit in pending for the next session. 

After 4:15pm ET, or for orders placed on a weekend/market holiday, market orders are accepted for execution at the next market open.