On November 2, 1983, President Ronald Reagan signed the King Holiday Bill into law, designating the third Monday in January a federal holiday in observance of civil rights leader Dr. Martin Luther King, Jr. The legislation to recognize Martin Luther King Jr. Day was first introduced just four days after his assassination on April 4, 1968. Still, it would take 15 years of persistence by civil rights activists for the holiday to be approved by the federal government and an additional 17 years for it to be recognized in all 50 states. Today, it is the only federal holiday designated as a national day of service to encourage all Americans to volunteer and improve their communities.
source: https://nmaahc.si.edu/explore/stories/15-year-battle-martin-luther-king-jr-day