Armed Forces Day was first celebrated in 1950. 

Since then, it has been recognized annually on the 3rd Sat. in May. 

The United States Armed Forces were unified under the Department of Defense following World War II. In 1949, the Secretary of Defense L. Johnson announced the creation of a unified Armed Forces Day to replace separate celebrations by the five U.S. service branches: Army, Navy, Marine Corps, Air Force, and Coast Guard. The U.S. Dept. of Defense Armed Forces Day website, "President Harry S. Truman led the effort to establish a single holiday for citizens to come together and thank our military members for their patriotic service in support of our country."