Decoration Day started in the South immediately after the Civil War when freedmen (former slaves) decorated the graves of Union soldiers who had been buried there...sometimes in unmarked graves after bloody battles. The freedmen were moved by gratitude for the sacrifice that had freed them. The practice caught on in the North a few years later and eventually morphed into Memorial Day which recognizes all those who gave their lives in war. It is the only day of commemoration set aside for those who paid the ultimate price. Veterans Day recognizes all who served their country. The Presidio always has a nice commemoration. I recommend it to those who have never attended. BTW, one should never say "Happy Memorial Day" any more than one would say "Happy Yom Kippur" or "Happy Good Friday". It's a day of reflection and gratitude.
Last Memorial Day I was in Western Massachusetts researching my ancestry when I came across a cemetery in Hatfield, MA and discovered that the local Boy Scouts always place flags on the graves of veterans on that day. The flags remain in place until Veterans Day, when the scouts take them down. I was surprised to learn that I had a great, great uncle who had died in the Civil War and two others who were veterans of that conflict. (Of course, the custom of decorating graves of soldiers goes back before the Roman empire.)