Originally called "Decoration Day" before WW11. The day started as an event to honor union soldiers who died during the American civil war. Inspiration taken from people in the southern states by the way they honored their dead.
After WW1 , it was extended to honor all men and women who died in any war or military action.
history of the day and much more from - http://www.timeanddate.com/holidays/us/memorial-day