How did Memorial Day become a national holiday?
It became a memorial day in 1868!
\Memorial Day ws adopted as an official holiday on a state-by-state basis, and was an official holiday in most states of the union by 1890. It began after the Civil War, and was originally known as "Decoration Day". "Memorial Day" started to be used more in the early years of the 20th century, and by the post WW II years, it was universally known as Memorial Day.