Abraham Lincoln made Thanksgiving a national holiday. This is from his 1863 proclamation, issued just a few days after Lincoln delivered the Gettysburg Address:
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. And I recommend to them that, while offering up the ascriptions justly due to him for such singular deliverances and blessings, they do also, with humble penitence for our national perverseness and disobedience, commend to his tender care all those who have become widows, orphans, mourners, or sufferers in the lamentable civil strife in which we are unavoidably engaged, and fervently implore the interposition of the almighty hand to heal the wounds of the nation, and to restore it, as soon as may be consistent with the Divine purposes, to the full enjoyment of peace, harmony, tranquillity, and union.
My former NRO colleague James Robbins discussed the Civil War and Thanksgiving here.