From the Queens Gazette:
Three years after the Civil War ended, on May 5, 1868, the head of an organization of former Union soldiers and sailors - the Grand Army of the Republic (GAR) - established Decoration Day as a time for the nation to decorate the graves of the war dead with flowers. Maj. Gen. John A. Logan declared it should be May 30. The first large observance was held that year at Arlington National Cemetery, across the Potomac River from Washington, D.C. The cemetery already held the remains of 20,000 Union dead and several hundred Confederate dead.
The ceremonies centered around the mourning-draped veranda of the Arlington mansion, once the home of Gen. Robert E. Lee. Gen. and Mrs. Ulysses S. Grant and other Washington officials presided. After speeches, children from the Soldiers' and Sailors' Orphan Home and members of the GAR made their way through the cemetery, strewing flowers on both Union and Confederate graves, reciting prayers and singing hymns.