This is Abraham Lincoln's October, 1864 Presidential Proclamation of Thanksgiving, 1864.
The roots of the national (not regional, or local) "official" celebration of Thanksgiving in the Civil War date back even earlier,however.
Jefferson Davis, the President of the Confederate States of America, made a declaration of Thanksgiving in 1861.
Back in 2011, I wondered how the first Civil War Thanksgiving (1861, our first year at war with each other) was celebrated by the troops on both sides.
In1861 there was no national Thanksgiving. Every state set its own date.
Thanks to another blogger, we have a good description of how Thanksgiving was celebrated by the troops in 1861. Some of the foods are those we would gobble (pun intended) down today.
The troops had a lot to be thankful about. Similar to the Thanksgiving dinner our troops get today (if at all possible, according to location) the troops on the designated Thanksgiving Day for their locality got special rations, and the chance to eat "real food": turkey, potatoes, and even oysters.
Recently, the New York Times published an article about the Civil War origins of our modern United States Thanksgiving. It is interesting reading, especially the part about the city of Petersburg, Virginia and their Thanksgiving - under siege by Union forces. Like so much about the Civil War, there is nothing simple about the origins of Thanksgiving.