Typically, the American flag flies at half-staff or at half-mast when the country or a state is in mourning. The president or a state governor (or D.C. Mayor) can order flags to fly at half-staff. In most cases, an American flag flying at half-staff marks one of three observances: - The death of a government official, military member, or emergency first responder - A national tragedy - Memorial Day or another national day of remembrance

Back in 1954, President Eisenhower issued a proclamation on the display of the flag, 19 FR 1235, 3 CFR. Whether everyone follows that is another matter.