Martin Luther King, Jr. Day is a United States federal holiday marking the birthday of Rev. Dr. Martin Luther King, Jr. It is observed on the third Monday of January each year, which is around the time of Dr. King's birthday, January 15.
This federal holiday celebrates the life and achievements of Martin Luther King Jr., an influential American civil rights leader. He is most well-known for his campaigns to end racial segregation on public transport and for racial equality in the United States. His most famous address was the "I Have A Dream" speech, and he became the youngest man to be awarded the Nobel Peace Prize.
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 on January 20, 1986. 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.
The holiday is seen as a day to promote equal rights for all Americans, regardless of their background. In recent years, federal legislation has encouraged Americans to give some of their time on this day as volunteers in citizen action groups. Many take the opportunity to celebrate Dr. Martin Luther King Jr. by observing his legacy of activism, awareness, service, and respect.
http://thekingcenter.org/about-dr-king/ Source: History of the King Center