I am not always a fan of Snopes.com, but they have a lengthy article on those who have rights to claim the beginnings of Decoration/Memorial Day. http://www.snopes.com/military/memorialday.asp
Being a military brat, both Decoration Day (what my family called it when I was a child) and the Fourth of July were very special observances. May 30 always meant gathering all of our peonies, irises, and Sweet William blossoms and heading to the cemetery.