Answer: The tradition of playing and watching football games on thanksgiving day first started in either 1876 or 1934, people are not exactly sure which year.
shortly after football had been invented, as it was a day that most people had off from work. In that year, the college football teams at Yale and Princeton began an annual tradition of playing each other on Thanksgiving Day.
It was kind of a random thing that one person started then everyone kind of just followed along.
Hope I helped!