This song appeared on Stevie Wonder's 1980 CD "Hotter Than July" (which is a GREAT CD by the way). The CD has a heavy reggae influence thanks to Stevie spending a lot of time with Bob Marley before he went into the studio.
At the time Stevie was very active in the campaign to make Martin Luther King Jr's birthday a Federal holiday. The holiday was very controversial.
In 1983 President Reagan signed the bill making it a holiday. The celebration started in 1986. However, not every state celebrated the holiday at first. For the longest time Arizona refused to. In Mississippi and Virgina the holiday was combined with the celebration of Confederate heroes Stonewall Jackson and Robert E. Lee.
Virgina finally split the holidays after many complained it wasn't very appropriate.
Happy Birthday Dr. Martin Luther King.