Although Ronald Reagan signed the holiday into law in 1983, Martin Luther King, Jr Day was not celebrated until 1986; the first observance was 20 Jan 1986. It is observed on the third Monday of January, like holidays set under the Uniform Monday Holiday Act.
The Rainbow Coalition was a multicultural movement founded April 4, 1969 in Chicago by members of the Black Panther Party (a revolutionary socialist political organization), the Young Patriots Organization (a left-wing organization) and the Young Lords (a civil- and human-rights organization). Whether intentional or not, it was founded on the first anniversary of Martin Luther King, Jr's assassination. Reverend Jesse Jackson eventually appropriated the phrase "rainbow coalition" in forming his own coalition, Rainbow/PUSH.