Dr. Gloria E. White-Hammond will serve as the keynote speaker at Arlington’s annual Martin Luther King Jr. birthday observance. White-Hammond is a physician as well as a founder and copastor of Bethel African Methodist Episcopal Church in Jamaica Plain. The interfaith celebration will be held in Town Hall on Jan. 21, with the free program starting at 7:30 p.m. The doors will open at 6:45 p.m. for a potluck dessert; participants encouraged to bring something sweet to share with others. A free-will collection will be taken to benefit local programs that further King’s goals of achieving equality and justice through nonviolent means.