U.S. holiday. Originally held (1868) in commemoration of soldiers killed in the American Civil War, its observance later extended to all U.S. war dead. Most states conform to the federal practice of observing it on the last Monday in May, but some retain the traditional day of celebration, May 30. National observance is marked by the placing of a wreath on the Tomb of the Unknowns in Arlington National Cemetery. Flags, insignia, and flowers are placed on the graves of veterans in local cemeteries.