In 1986, a national holiday was created to celebrate the life and work of Baptist minister and civil rights activist Martin Luther King Jr. Ever since then, children and adults alike celebrate MLK day on the third Monday of January to honor his birthday.
Martin Luther King Jr. was born on January 15, 1929. In 1955, he rose to prominence from an African-American bus boycott in Montgomery, Alabama. He continued serving Black people of America by founding the Southern Christian Leadership Conference in 1957. Famously, King led a march on the Capitol in 1963 to protest racial discrimination. King’s advocacy gave rise to the passage of the Civil Rights Act of 1964, which outlaws discrimination in all public facilities, as well as the Voting Rights Act of 1965, an act that outlaws voting practices that racially discriminate. Because of his work, King was awarded the Nobel Peace Prize in 1964.
MLK was assassinated on April 4, 1968, at just 39 years old. However, his legacy lives on, and there was an immediate push for a national holiday in his honor by congressman John Conyers. Congress passed legislation in 1983 that made the third Monday in January a federal holiday, and it was first observed in 1986.

Britannica
https://www.history.com/news/martin-luther-king-jr-day-controversial-origins-of-the-holiday
https://constitutioncenter.org/blog/how-martin-luther-king-jr-s-birthday-became-a-holiday-3