Well, on Nov 11, we observe Veterans' Day. It used to be called Armistice Day, marking the end of WW1, but there have been other wars since then, and so it became a day to honor all military veterans.
Memorial Day had its origin after the American Civil War, and began as Decoration Day, when the graves of the Civil War dead were decorated every May 30. Later on, again because there were more wars, it became Memorial Day, honoring the dead of all American wars. More recently (about 1970), the day was changed to the last Monday in May, rather than a fixed date.