Martin Luther King Jr. Day (officially Birthday of Martin Luther King, Jr., and sometimes referred to as MLK Day) is a federal holiday in the United States marking the birthday of Martin Luther King Jr. It is observed on the third Monday of January each year. King's birthday is January 15. The holiday is similar to holidays set under the Uniform Monday Holiday Act. The earliest Monday for this holiday is January 15 and the latest is January 21.

King was 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 in 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.