National POW-MIA Recognition Day, one of six days each year that Postal Service facilities are required to fly the POW-MIA flag, is Friday, Sept. 16.
The flag honors those who were prisoners of war and those who are missing in action.
All USPS facilities must display the POW-MIA flag. The POW-MIA flag must fly below the U.S. flag.
Source: USPS News Link