The first American thanksgiving was held at Jamestown and similar thanksgivings were celebrated in several colonies. The official establishment of Thanksgiving Day was, therefore, not an act of unitarians or deists to replace Christmas but an extension of something that had already been going on.and:
America was never a Calvinist project until the War between the States. That is just a typical Yankee lie. The Puritans had never liked Christmas to begin with. There are no American founders because there was no American founding--the Articles of Confederation and Constitution were documents solidifying a confederation of sovereign states and therefore any references to God, religion, the family, marriage, would have been entirely out of order. The First Amendment was not a defense against deism--it was more or less drafted by Madison!--but against any infringement into the rights of the states in religious matters. The Yankees had been alarmed by the Quebec Act, which granted religious freedom to Catholics not only in Quebec but in NW territories partly claimed by Connecticut. They also feared, or said they did, the imposition of the Cof E on the colonies. The First Amendment, like the other provisions in the Bill of Rights, were not statements of lofty principles but responses to some of the crises that brought on the Revolution and to current circumstances. It should be pointed out that no one in America much liked the Yankee Puritans, not even the decent folk of New York and Pennsylvania which were being overrun by Yankee rabble. Read Cooper's Satanstoe novels or The American Democrat.
There was a wide variety of religious establishment in the states, ranging from requirements that office-holders believe in God to using tax money to support the clergy--but in a mobile and complex population, establishments were no more practical than monarchy or aristocracy.
America was "founded" either as a religious country or a secular country, and if one can prove it was not a religious foundation, then it must be secular. But the dichotomy itself is false and puerile. There is no American founding, no American ideology, only a vague set of assumptions that varied by region. As a number of historians have shown (David Hackett Fisher, M.E. Bradford), there was no uniform America in the 19th century but a set of regions with differing cultures derived from the different parts of Britain for which the predominant elements came. A single state like South Carolina was home to several cultures--SW England by way of Barbados, Scoth-Irish, Huguenon, Catholic Irish.
M. E. Bradford, Founding Fathers