Our Flag
Fly the American flag at half-staff
The United States flag flies at half-staff or at half-mast when the nation or a state is in mourning. The president, a state governor, or the mayor of the District of Columbia can order flags to fly at half-staff. Most often, it marks:

The death of a government official, military member, or first responder
A national tragedy
Memorial Day or another national day of remembrance