Martin Luther King Jr. Day, officially known as the Birthday of Martin Luther King, Jr., and commonly referred to as MLK Day, is a federal holiday in the United States commemorating the birth of Martin Luther King Jr. This observance takes place on the third Monday of January each year. Although King was born on January 15, 1929 (a Tuesday in that year), the holiday is set according to the Uniform Monday Holiday Act. It can fall on any Monday between January 15 and January 21.

As a key advocate for nonviolent activism in the Civil Rights Movement, King played a pivotal role in challenging racial discrimination in federal and state laws. The initiative to establish a federal holiday in King's honor gained momentum following his assassination in 1968. President Ronald Reagan signed the holiday into law in 1983, and its inaugural observance occurred on January 20, 1986. Initially, some states hesitated to recognize the holiday, offering alternative names or merging it with other observances. However, by the year 2000, it became officially observed in all 50 states.