Memorial Day, then Decoration Day, did arise from the Civil War

Decordation Day was pushed by General John ("Black Jack") Logan after the Civil War concluded. Logan, then a Democrat, was a "political" general, who was engaged in the Western theater, including Vicksburg. Logan would become a Rupublican, and was James Blaine's running mate in an election lost by the Republicans. The interplay of the Gettysburg Address and Thanksgiving is a story for later.