According to the info posted with the below video,

On April 4, 1969, the first anniversary of the death of Dr. Martin Luther King, the third night of Passover, hundreds of people of varied racial and religious communities gathered in a Black church in the heart of Washington DC to celebrate the original Freedom Seder. For the first time, it intertwined the ancient story of liberation from Pharaoh with the story of Black America’s struggle for liberation, and the liberation of other peoples as well.

More info on the event can be found here.