Memorial Day was not declared a national holiday until 1971 but it has been celebrated in many areas of the United States unofficially since 1868 when it started as a day to remember soldiers who had died in the Civil War or War Between The States as some would say.
Poppies became a symbol of Memorial Day shortly after World War One and partly from the effort of Moina Michael who was inspired by this line from John McCrae's poem, In Flanders Field:
To you from failing hands we throw the Torch; be yours to hold it high. If ye break faith with us who die, we shall not sleep, though poppies grow in Flanders Fields."
There is more good information about Memorial Day at the PBS National Memorial Day Concert website. I enjoy watching this concert and it is a good family event with great music.
I suggest you also check out one of my favorite music blogs Music You (Possibly) Won't Hear Anyplace Else. The author, Lee, has put together an incredibly collection of patriotic music to download and some of it is spoken word. Lee is himself a veteran of the Navy so stop by, download some great music and thank him for his service to our country!