Six years ago, President Obama -- then a senator from Illinois -- spoke at the groundbreaking ceremony for the Martin Luther King, Jr. Memorial. 

He described a man who, though not a president, became the leader of a nation. He described a man who, though he frequently wrestled with doubt, gave voice to the voiceless and courage to the faint of heart. 

And the President described the monument as a reminder that King's dream of "a land in which all of God's children might come together in a spirit of brotherhood" still beckons. 

In the time since, President Obama has compared Dr. King to Moses -- a visionary leader who did not live to see the Promised Land. 

In 2010, the President spoke about King at the Vermont Avenue Baptist Church in Washington, D.C., describing himself as a member of the Joshua generation, "the great inheritors of progress paid for with sweat and blood, and sometimes life itself."

That same year, the President discussed an idea that he had heard from the historian Taylor Branch, that Dr. King's birthday should not just be "a time to celebrate service, to reflect and study on how we had helped to perfect our union, but that it should be a day in which each of us individually also try to stretch out of our comfort zones and try to do something for others and to reach out and learn about things that maybe we've shied away from."On Sunday, the President will again speak at the space devoted to the civil rights pioneer. The MLK Memorial opened in August, but Hurricane Irene delayed the dedication of the site -- until now. At the event, the President will be joined by civil rights and religious leaders, as well as poets and musicians.

PHOTO: President Barack Obama, First Lady Michelle Obama, daughters Sasha and Malia, and Marian Robinson tour the Martin Luther King Jr. National Memorial before the dedication ceremony in Washington, D.C., Sunday, Oct. 16, 2011. (Official White House Photo by Pete Souza