How did Thanksgiving become an American National Holiday?
President Abraham Lincoln declared a National Holiday for the United States of America in 1863, with the following “Thanksgiving Proclamation”. It’s amazing, 147 years later and some things still haven’t change.

President Abraham Lincoln’s 1863 Thanksgiving Proclamation:
“It is the duty of nations as well as of men to own 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.

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 ever 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-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 American people. I do therefore invite my fellow citizens in every part of the United States, and also those who are at the 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.”

A. Lincoln, October 3, 1863