Purim
The one feast in the calendar that nobody commanded — kept because a massacre did not happen.
Purim is named after the lot Haman cast to pick the day for destroying the Jews of Persia, and it is kept on the day the destruction was to have happened. Unlike every other feast here it comes from no law: Esther and Mordecai establish it, and the book records the people taking it on themselves. That makes it the only appointed day in the Hebrew Bible instituted by the people rather than given to them.
