According to the Washington Post:
On May 1, 1865, thousands of newly freed Black people gathered in Charleston, S.C., for what may have been the nation’s first Memorial Day celebration. Attendees held a parade and put flowers on the graves of Union soldiers who had helped liberate them from slavery.
But the day would not be remembered as the first Memorial Day. In fact, White Southerners made sure that for more than a century, the day wasn’t remembered at all.
It was “a kind of erasure from public memory,” said David Blight, a history professor at Yale University.