THANKSGIVING DAY 1863
By the President of the United States : a Proclamation
...In the midst of a civil war of unequaled magnitude and severity, which has sometimes seemed to foreign states to invite and to provoke their aggression, peace has been preserved with all nations, order has been maintained, the laws have been respected and obeyed, and harmony has prevailed everywhere, except in the theater of military conflict, while that theater has been greatly contracted by the advancing armies and navies of the Union...
...I do therefore invite my fellow-citizens in every part of the United States, and also those who are at sea and those who are sojourning in foreign lands, to set apart and observe the last Thursday of November next as a day of thanksgiving and praise to our beneficent Father who dwelleth in the heavens.
ABRAHAM LINCOLNSOURCE: Pilgrim Hall Museum website, which lists a sample of wartime proclamations from 1898, 1917, 1943, 1953, 1967 and 1991.