The official history of Memorial Day goes like this: In 1868, three years after the Civil War ended, a group of Union veterans established a Decoration Day for the nation to adorn the graves of the war dead with flowers. A retired Union major general, John A. Logan, set the date of the holiday for May 30, and the holiday’s first observance was at Arlington National Cemetery.

David W. Blight, a historian at Yale, has a different account. He traces the holiday to a series of commemorations that freed black Americans held in the spring of 1865, after Union soldiers, including members of the 21st United States Colored Infantry, liberated the port city of Charleston, S.C.

Digging through an archive at Harvard, Dr. Blight found that the largest of these commemorations took place on May 1, 1865, at an old racecourse and jockey club where hundreds of captive Union prisoners had died of disease and been buried in a mass grave. The black residents exhumed the bodies and gave them proper burials, erected a fence around the cemetery, and built an archway over it with the words, “Martyrs of the Race Course.”

Some 10,000 black people then staged a procession of mourning, led by thousands of schoolchildren carrying roses and singing the Union anthem “John Brown’s Body.” Hundreds of black women followed with baskets of flowers, wreaths and crosses. Black men, including Union infantrymen, also marched. A children’s choir sang spirituals and patriotic songs, including “The Star-Spangled Banner.”

“The war was over, and Memorial Day had been founded by African-Americans in a ritual of remembrance and consecration,” Dr. Blight wrote in a 2011 essay for The New York Times. “The war, they had boldly announced, had been about the triumph of their emancipation over a slaveholders’ republic. They were themselves the true patriots.”The African-American origins of the holiday were later suppressed, Dr. Blight found, by white Southerners who reclaimed power after the end of Reconstruction and interpreted Memorial Day as a holiday of reconciliation, marking sacrifices — by white Americans — on both sides. Black Americans were largely marginalized in this narrative.