The Super Bowl is the championship game of the National Football League (since 1967), the premier association of professional American football. In most years, the Super Bowl is the most-watched American television broadcast. Many popular singers and musicians have performed during the event’s pre-game and halftime ceremonies. The day on which the Super Bowl is played is now considered to be a de facto American national holiday, called Super Bowl Sunday. Super Bowl Sunday is the second-largest U.S. food consumption day, after Thanksgiving Day. Exclusive television broadcast rights for the Super Bowl rotate each year among 3 of the 4 major American television networks: CBS, NBC and Fox. Because of its high viewership, commercial airtime for the Super Bowl broadcast is the most expensive of the year. Due to the high cost of investing in advertising on the Super Bowl, companies regularly develop their most expensive (and ostensibly, best) advertisements for this broadcast. As a result, watching and discussing the broadcast’s commercials has become a significant aspect of the event as well.The Super Bowl was first played on January 15, 1967, as part of a merger agreement between the NFL and a rival league, the American Football League (AFL). It was agreed that the two leagues’ champion teams would play in an AFL–NFL World Championship Game, until the merger was consummated. After the merger of the two leagues in the 1970s, each league became a "conference", and the game was played between conference champions. Lamar Hunt, former owner of the Kansas City Chiefs and founding member of the American Football League, coined the name Super Bowl after watching his children playing with a Super Ball. The Super Bowl uses Roman numerals to identify each game, rather than the year in which it is held. Super Bowl I was played in 1967 to determine the championship of the regular season played in 1966, and Super Bowl XLV will be played in 2012 to determine the champion of the 2010 regular season.The Dallas Cowboys have appeared in the greatest number of Super Bowls (8). With five victories, the Pittsburgh Steelers have won more Super Bowls than any other franchise; seventeen of the NFL's 32 teams have won at least one Super Bowl. Only five active NFL franchises have not appeared in the Super Bowl. They are the Detroit Lions, Cleveland Browns, New Orleans Saints, Jacksonville Jaguars, and Houston Texans. The Browns and Lions won NFL championships in the pre-Super Bowl era; the Saints, Jaguars, and Texans have all joined the league after the kickoff of the Super Bowl era.