The first Memorial Day-type commemorations were observed during and after the Civil War. In 1866, the Grand Army of the Republic organization proclaimed that May 30 of that year should be observed as Decoration Day, and by 1890, Decoration Day was an official holiday in every northern state. In 1967, Decoration Day became Memorial Day by Federal law.
From Oliver Wendell Holmes, Jr’s 1884 Memorial Day Address - In Our Youth Our Hearts Were Touched with Fire:
So to the indifferent inquirer who asks why Memorial Day is still kept up we may answer, it celebrates and solemnly reaffirms from year to year a national act of enthusiasm and faith. It embodies in the most impressive form our belief that to act with enthusiam and faith is the condition of acting greatly. To fight out a war, you must believe something and want something with all your might… More than that, you must be willing to commit yourself to a course, perhaps a long and hard one, without being able to foresee exactly where you will come out. All that is required of you is that you should go somewhither as hard as ever you can. The rest belongs to fate.