Flags are often flown at half-staff during times of mourning over the loss of a significant figure or to commemorate a significant anniversary. The length of time that a flag is at half-staff depends on the significance of the person who has recently passed away.
In the United States, flags must be at half-staff for 30 days after a former or current president has died. This tradition began during the early 1600s, when British sailors lowered the flag on their ship to pay respect to a fallen captain. In addition to paying respect to a well-known figure, flags are typically flown at half-staff for special holidays, such as Patriot Day, Peace Officers Memorial Day and Pearl Harbor Remembrance Day.