Generally, securities companies are allowed to entrust the pending orders for tomorrow's trading one night in advance. General securities companies accept the pending orders for the next transaction after the liquidation on the same day. Generally, the liquidation on the same day is completed after 9: 30 pm. Note that securities companies may not accept relevant stock pending orders on the last trading day before Saturday, Sunday or holidays, and these pending orders may not be entrusted before Monday morning.
The main reason is that when the stock market is closed on Saturday, Sunday and holidays, securities companies often conduct system tests. In order to prevent data confusion, they are generally not entrusted to hang orders to prevent confusion between test data and real entrusted data.
Reference: Baidu Encyclopedia-Stock Pending Orders