Martin Luther King Jr. Day (officially Birthday of Martin Luther King, Jr., and sometimes referred to as MLK Day) is an American federal holiday marking the birthday of Martin Luther King, Jr. It is observed on the third Monday of January each year. King’s actual date of birth was January 15, 1929. King was an American Christian minister and the chief spokesperson for nonviolent activism in the Civil Rights Movement, which successfully protested racial discrimination in federal and state law. The campaign for a federal holiday in King’s honor began soon after his assassination on April 4, 1968. President Ronald Reagan signed the holiday into law in 1983, and it was first observed three years later. At first, some states resisted observing the holiday as such, giving it alternative names or combining it with other holidays. It was officially observed in all 50 states for the first time in 2000.
Source: https://en.wikipedia.org/wiki/Martin_Luther_King_Jr.