One-eighth of the whole population were colored slaves, not distributed generally over the Union, but localized in the southern part of it. These slaves constituted a peculiar and powerful interest. All knew that this interest was somehow the cause of the war. To strengthen, perpetuate, and extend this interest was the object for which the insurgents would rend the Union even by war, while the Government claimed no right to do more than to restrict the territorial enlargement of it. Neither party expected for the war the magnitude or the duration which it has already attained. Neither anticipated that the cause of the conflict might cease with or even before the conflict itself should cease.
It was originally known as Decoration Day, and arose out of the practice of decorating the graves of dead soldiers on both sides of the conflict. From the Wikipedia article:
The Memorial Day speech became an occasion for veterans, politicians and ministers to commemorate the war--and at first to rehearse the atrocities of the enemy. They mixed religion and celebratory nationalism and provided a means for the people to make sense of their history in terms of sacrifice for a better nation, one closer to God. People of all religious beliefs joined together, and the point was often made that the Germans and Irish soldiers had become true Americans in the "baptism of blood" on the battlefield. By the end of the 1870s the rancor was gone and the speeches praised the brave soldiers both Blue and Gray. By the 1950s, the theme was American exceptionalism and duty to uphold freedom in the world.To commemorate this day and the war in which it originated, I am linking this video. The volume is a little low, so you will have to turn it up.