Only at the point of a bayonet, because the North won the war. If the South had won its independence, that the north denied, there would have most likely been a different day set aside.

Remember, the Episcopal Church had been celebrating Thanksgiving on the 1st day of November for decade prior to the War for Southern Independence. Oddly enough the 1st of November was already at that time a religious observance anyway called All Saints Day, the day after All Hallows Eve.