Final answer:
The first Thanksgiving football game was played between Yale and Princeton on November 30, 1876. It signified the beginning of football becoming a Thanksgiving tradition, a concept which greatly predates the NFL.
Step-by-step explanation:
The first Thanksgiving football game took place on November 30, 1876, establishing a tradition of football on Thanksgiving Day. The game was played between Yale and Princeton. The concept of Thanksgiving football games predates the NFL and became a part of the holiday's celebration as a result of these college football matches which were already popular during the latter part of the 19th century. College football rivalries, like the one between Yale and Princeton, were legendary by the time the first Rose Bowl was played in 1902.
While the first Thanksgiving celebration in Plymouth in 1621 included feasting and perhaps games, it did not involve football, which didn't come into existence until much later. The 1621 event, detailed by Edward Winslow in Mourt's Relation, included a three-day feast with the Pilgrims and Native Americans; however, contemporary celebrations of Thanksgiving often include watching or playing football as a staple American tradition.