National POW/MIA Recognition Day is celebrated on September 21, 2018. In the United States, National POW/MIA Recognition Day is observed on the third Friday in September. It honors those who were prisoners of war and those who are still missing in action. This day was established by an Act of Congress, by the passage of Section 1082 of the 1998 Defense Authorization Act.
It is one of six days that the POW/MIA Flag can be flown. The POW/MIA flag was first recognized by Public Law 101-355 in 1990. The POW/MIA flag should fly below, and not be larger than the United States flag. It is generally flown immediately below or adjacent to the United States flag as second in order of precedence. (With material from: Wikipedia)