Although this time of year was always the harvest, and wise people are naturally grateful for their bounty and sustanance from the earth/almighty deity of their choice, this is what alot of scholars and historians consider the basis for the modern 'holiday' of Thanksgiving:

THE ORIGINAL DOCUMENT_ 1863>>

Proclamation of Thanksgiving, by Abraham Lincoln.
Washington, D.C.
October 3, 1863
By the President of the United States
A Proclamation.

The Year that is drawing to a close, has been filled with the blessings of fruitful fields and healthful skies. To these bounties, which are so constantly enjoyed that we are prone to forget the source from which they come, others have been added, which are so extraordinary a nature, that they cannot fail to penetrate and soften even the heart which is habitually insensible to the ever watchful providence of Almighty God.
In the midst of a civil war of unequaled magnitude and severity, which has sometimes seemed to foreign States to invite and to provoke the 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.
Needful diversion of wealth and strength from the fields of peaceful industry to the national defense, have not arrested the plough, the shuttle or the ship; the axe has enlarged the borders of our settlements, and the mines, as well of iron and coal as of the precious metals, have yielded even more abundantly than heretofore.
Population has steadily increased, notwithstanding the waste that has been made in the camp, the siege, and the battle-field; and the country, rejoicing in the consciousness of augmented strength and vigor, is permitted to expect continuance of years with large increase of freedom.
No human counsel hath devised nor hath any mortal hand worked out these great things. They are the gracious gifts of the Most High God, who, while dealing with us in anger for our sins, hath nevertheless remembered mercy.
It has seemed to me fit and proper that they 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 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 ascription's justly due to Him for such singular deliverance's 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, tranquility, and Union.
In testimony whereof, I have hereunto set my hand and caused the seal of the United States to be affixed.
Done at the City of Washington, this Third day of October, in the year of our Lord one thousand eight hundred and sixty-three, and of the Independence of the United States the Eighty-eighth.

By the President: Abraham Lincoln