Martin Luther King Day
noun
Mar·tin Lu·ther King Day ˈmär-tᵊn-ˈlü-thər-ˈkiŋ- 
: the third Monday in January observed as a legal holiday in the U.S.
Note: The federal name for this holiday according to the United States Code is Birthday of Martin Luther King, Jr.
Word History
First Known Use
1968, in the meaning defined above
Kids Definition
Martin Luther King Day
noun
Mar·tin Lu·ther King Day ˈmär-tᵊn-ˈlü-thər-
: the third Monday in January observed as a legal holiday in the United States