Definition of Martin Luther King Jr. Day noun from the Oxford Advanced American Dictionary

Martin Luther King Jr. Day

noun
/ˌmɑrtn ˌluθər ˌkɪŋ ˈdʒunyər ˌdeɪ/
 
(also Martin Luther King Day
/ˌmɑrtn ˌluθər ˈkɪŋ deɪ/
 
)
 
a national holiday in the U.S. on the third Monday in January to celebrate the birthday of Martin Luther King, Jr., who was active in the struggle to win more rights for Black Americans