How did Thanksgiving Day become official?
Several presidents proclaimed individual days of thanksgiving for God's goodness to them, starting with George Washington.
Abraham Lincoln realized the need for Americans to turn back to God with a full heart. He gave this address to announce the new annual American holiday.
Lincoln's Thanksgiving Address
It is the duty of nations as well as of men to owe their dependence upon the overruling power of God; to confess their sins and transgressions in humble sorrow, yet with assured hope that genuine repentance will lead to mercy and pardon; and to recognize the sublime truth, announced in the Holy Scriptures and proven by all history, that those nations are blessed whose God is the Lord.
We know that by his divine law, nations, like individuals, are subject to punishments and chastisements in this world May we not justly fear that the awful calamity of civil war which now desolates the land may be a punishment inflicted upon us for our presumptuous sins; to the needful end of our national reformation as a whole people?
We have been the recipients of the choicest bounties of heaven; we have been preserved these many years in peace and prosperity; we have grown in numbers, wealth and power as no other nation has grown.
But we have forgotten God. We have forgotten the gracious hand which preserved us in peace and multiplied and enriched and strengthened us, and we have vainly imagined, in the deceitfulness of our hearts, that all these blessings were produced by some superior wisdom and virtue of our own.
Intoxicated with unbroken success we have become too self-sufficient to feel the necessity of redeeming and preserving grace, too proud to pray to the God that made us.
It has seemed to me fit and proper that God should be solemnly, reverently, and gratefully acknowledged, as with one heart and one voice, by the whole of the American people. I do therefore invite my fellow citizens in every pad 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 as a day of Thanksgiving and praise to our beneficent Father who dwelleth in the Heavens.
-President Abraham Lincoln
16th president of the USA