Memorial Day, originally called Decoration Day, began in the United States after the Civil War to honor soldiers who died in the conflict. The first widely recognized observance was on May 30, 1868, when General John A. Logan, leader of the Grand Army of the Republic, a Union veterans’ organization, called for a nationwide day to decorate soldiers’ graves with flowers. This date was chosen because it wasn’t tied to any specific battle, and flowers would bloom.
Source: Grok
The flowers referenced by Grok were the spring roses in my great-great-grandmother's garden. She was John A. Logan's cousin and suggested the flower connection when General Logan told her he wanted to choose the proper date for the holiday.
My great-great-grandfather served in the Union Army and was captured by the Confederates. He was eventually shipped to the notorious POW camp Andersonville. One day, the guards lined the prisoners up and picked several, including great-great-grandad, out of the crowd. He thought the Union prisoners were going to be shot, but they had been chosen to be orderlies in the nearby rebel hospital. Great-Great-Grandad said he wasn't much for taking care of rebs, but at least they feed us.
That random act may have saved his life. Memorial Day is meant to honor all those who were not as lucky as he was.