Probably held in late September or early October, the first Thanksgiving was likely based on the traditional English Harvest Festival, as well as a communal bonding between the English and the Pokonokets, led by Massasoit, since forming a mutually-beneficial alliance in March 1621 – following a very rough year of English suffering, fear, anxiety, and death, and a number of devastating years of Pokonoket disease and blight.
The first Plymouth Colony (a joint entrepreneurial/religious venture, and one of the earliest successful English colonies in North America) had survived one year, becoming the embryonic form of what would become the United States of America. One of the keys to this survival was the Mayflower Compact, which served as the first governing document of our emerging nation – a "social contract", forming a "body politic" – where men determined to follow the rule of law, agreeing to work together for mutual survival, not following the whim, or personal penchant, of any particular man.
The pragmatism, determination, and conviction of the original Pilgrims, particularly Edward Winslow and Stephen Hopkins (more here), formed the mold of a rugged American pioneering spirit; a spirit that isn’t afraid to blaze new trails, take risks, criticize authority when it feels it is unjustly treated, and speaks its mind freely, in the face of reprimand and admonishment.
They weren’t perfect – they were people, just like us, of their particular time and circumstance – but they are the prototypical form of who Americans are, why Americans are, and what Americans are.