via the United States Department of Defense - 'The History of Armed Forces Day':

On August 31, 1949, Secretary of Defense Louis Johnson announced the creation of an Armed Forces Day to replace separate Army, Navy and Air Force Days. The single-day celebration stemmed from the unification of the Armed Forces under one department -- the Department of Defense. Each of the military leagues and orders was asked to drop sponsorship of its specific service day in order to celebrate the newly announced Armed Forces Day. The Army, Navy and Air Force leagues adopted the newly formed day. The Marine Corps League declined to drop support for Marine Corps Day but supports Armed Forces Day, too.

In a speech announcing the formation of the day, President Truman "praised the work of the military services at home and across the seas" and said, "it is vital to the security of the nation and to the establishment of a desirable peace." In an excerpt from the Presidential Proclamation of Feb. 27, 1950, Mr. Truman stated: "Armed Forces Day, Saturday, May 20, 1950, marks the first combined demonstration by America's defense team of its progress, under the National Security Act, towards the goal of readiness for any eventuality. It is the first parade of preparedness by the unified forces of our land, sea, and air defense".