That what is now known as Memorial Day in the US, and serves as our Armistice Day does (remembrance, not the start of summer) was a post civil war Union tradition called Decoration Day first officially held at Arlington National Cemetery in 1868, itself the start of the American tradition of “repatriating” their war dead.
Garnered from “Death and the Civil War” on PBS, a programme that I suspect owes a large debt to Ken Burns’ “Civil War” documentary.