By late in the century nearly every element of a modern Thanksgiving was in place (including football games with somewhat different rules.) Only one thing was missing - the Pilgrims.
Scholars rediscovered accounts of the Pilgrim/ Wampanoag festival of 1621 and some pronounced it the “First Thanksgiving.” Some also cited the 1623 Plymouth feast that was called a “thanksgiving.”
As a colonial revival mood took hold - emphasizing charm and quaintness over historical accuracy - the Pilgrim story fit right in. By the twentieth century the familiar (but inaccurate) image of Pilgrims with black hats and large buckles was a gift shop/greeting card staple.