The first owner of the Lions, G.A. Richards, started the tradition of the Thanksgiving Day game as a gimmick to get people to go to Lions football games, and to continue a tradition begun by the city's previous NFL teams.[1] It is widely rumored that the Cowboys sought a guarantee that they would regularly host Thanksgiving games as a condition of their very first one (since games on days other than Sunday were uncommon at the time and thus high attendance was not a certainty).
Several other NFL teams played regularly on Thanksgiving in the past, including the Chicago Bears and Chicago Cardinals (1922-33; the Bears played the Lions from 1934 to 1939 while the Cardinals switched to the Green Bay Packers for 1934 and 1935), Frankford Yellow Jackets, New York Giants (1929-38, who always played a crosstown rival), and Green Bay Packers (1951-63, always as the away team to the Lions). In 1939 and 1940, during the Franksgiving controversy, the only two teams to play the game were the Pittsburgh Pirates and Philadelphia Eagles, as both teams were in the same state (Pennsylvania); at the time, then-president Franklin Roosevelt wanted to move the holiday purely for economic reasons and many states were resistant to the move, which would have complicated scheduling for the game.
The two "traditional" Thanksgiving Day pro football games were in Dallas and Detroit. Because of TV network commitments, to make sure that both the AFC-carrying network and the NFC-carrying network got at least one game each, one of these games was between NFC opponents, and one featured AFC-NFC opponents. Thus, the AFC could showcase only one team on Thanksgiving, and the AFC team was always the visiting team.