Dr. King was known for using nonviolent civil disobedience and protests to end segregation (the separation of citizens, in this case based on race), which was widespread at the time, especially in the Southern U.S. He helped organize the March in Washington in 1963, where he delivered his now famous "I Have A Dream" speech.
Dr. King received the Nobel Peace Prize in 1964 for his strategy of nonviolence in fighting racial inequality. He was assassinated in 1968 during a tumultuous (chaotic; disorderly) period in U.S. history. The federal holiday on Monday commemorates his birthday, which is on January 15. Watch this short video from History.com about that historic March in Washington, D.C. and Dr. King's speech, and enjoy your day off!
-- posted by Joe Yu, ESL instructor