All trades must be closed before 4:59 PM ET.
Ags must be closed earlier. To see the rules on when Ags close, see In Which Markets Must Trades Be Closed Earlier Than Before 4:59 PM ET?
(Ags is short for Agricultural.)
Keep in mind, some markets close earlier than 5:00 PM.
Also, remember some markets close early on holidays. Here is the current holiday calendar for your reference: Current Holiday Calendar
Starting at 6:00 PM ET, you can start placing trades again.