Memorial Day.

In the US, a day of remembrance (formerly Decoration Day) instituted 1868 for those killed in the US Civil War. Since World War I it has been observed as a national holiday on the last Monday in May, traditionally falling on 30 May, in remembrance of all Americans killed in war.
U.S., last Monday in May; commemorates the members of the United States armed forces who were killed in war; Also called: Decoration Day.