No centuries-long continuity emerged from that 1621 meet-up. New Englanders certainly celebrated Thanksgivings—often in both fall and spring—but they were of the fasting-and-prayer variety. Notable examples took place in 1637 and 1676, following bloody victories over Native people. To mark the second occasion, the Plymouth men mounted the head of Ousamequin’s son Pumetacom above their town on a pike, where it remained for two decades, while his dismembered and unburied body decomposed. The less brutal holiday that we celebrate today took shape two centuries later, as an effort to entrench an imagined American community. In 1841, the Reverend Alexander Young explicitly linked three things: the 1621 “rejoicing,” the tradition of autumnal harvest festivals, and the name Thanksgiving. He did so in a four-line throwaway gesture and a one-line footnote. Of such half thoughts is history made.
Philip Deloria, "The Invention of Thanksgiving." The New Yorker, 11/18/2019.