How do i know when to fly a flag at half mask?

A flag is flown at half-staff after the death of a President or a national tragedy IF requested by the President. Here is a list of days that we fly at half staff and rules for doing so.How do i know when to fly a flag at half mask?
Half Mast. Follow the lead of your local post office. Any time our nation is in mourning, is the best reference I can give you.