The United States flag flies at half-staff (or half-mast) when the nation or a state is in mourning. The president, through a presidential proclamation, a state governor, or the mayor of the District of Columbia can order flags to fly at half-staff. Most often, this is done to mark the death of a government official, military member, or first responder; in honor of Memorial Day or other national days of remembrance; or following a national tragedy.
Learn more from the Department of Veterans Affairs about the rules and traditions for flying the flag at half-staff (PDF).
Visit halfstaff.org for more information.