長假結束卻沒有按時回單位上班，給老板打電話說飛機取消了，要晚一天回去。這種事情恐怕不少人都經歷過吧。

Employees usually need to ask for permission from their boss for leave, which is called qingjia in Chinese. But on certain occasions like the first day of work after the Spring Festival, many employees fail to show up to work without asking for permission. Most cook up excuses like flight delays, leaving the boss no choice but to grant them the leave. Thus, a fait accompli leave: the leave is already taken, and there is no point denying it. If something is a fait accompli, it has already been decided or done and cannot be changed.
員工通常需要經過老板允許才能休假，這在中文里叫“請假”。但是，在一些特殊時期，比如春節過后的首個工作日，很多員工都沒有回來上班，但也沒有請假，大部分都以航班延誤等作為晚歸理由。老板無奈，也只能準假。這種情況就叫“霸王假”，人家既然都沒來上班，你否決也沒有意義了。fait accompli表示既成事實，也就是無法更改或撤銷的事實。

For example:

Nearly half of the staff didn't show up for work after the New Year holiday, it seems they all took fait accompli leave.
新年假期結束后有近一半的員工沒有來上班，貌似都請了“霸王假”啊。

（中國日報網英語點津 Helen）