From the U.S. Department of Defense website.
ARMED FORCES DAY
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.
On August 31, 1949, Secretary of Defense Louis Johnson announced the creation of an Armed Forces Day to replace separate Army, Navy, Marine Corps, and Air Force Days.
The single-day celebration stemmed from the unification of the Armed Forces under the Department of Defense.