The trade deadline is the last day of the regular season on which teams may trade players. After it, rosters change only through free agency until the season ends.
In HoopSim the trade deadline is February 6 on the league calendar by default, and the commissioner can move it. Trades are open in the offseason, staff free agency, the draft, free agency, training camp, preseason and the regular season up to the deadline. They close after it, through the play-in and the playoffs, and reopen when the offseason starts. A trade accepted just before the deadline but still in its 24-hour veto window after it will fail, so leave time. The trade block closes when trades do. Free agency does not close: minimum deals and in-season offers still work after the deadline.
Rulebook: Veto window and the deadline. Related: trade block, buyout.