Originally called Decoration Day, from the early tradition of decorating graves with flowers, wreaths and flags, Memorial Day is a day for remembrance of those who have died in service to our country. It was first widely observed on May 30, 1868 to commemorate the sacrifices of Civil War soldiers, by proclamation of Gen. John A. Logan of the Grand Army of the Republic, an organization of former Union sailors and soldiers.
During that first national celebration, former Union Gen. and sitting Ohio Congressman James Garfield made a speech at Arlington National Cemetery, after which 5,000 participants helped to decorate the graves of the more than 20,000 Union and Confederate soldiers who were buried there.
Via PBS: The History of Memorial Day