According to this site, it was "The Wizard of Oz" (1939) on 3rd November 1956.
1950s FILM HISTORY
Film attendance declined precipitously as free TV viewing (and the increase in popularity of foreign-language films) made inroads into the entertainment business. In 1951, NBC became America's first nationwide TV network, and in just a few years, 50% of US homes had at least one TV set. In March of 1953, the Academy Awards were televised for the first time by NBC - and the broadcast received the largest single audience in network TV's five-year history. By 1954, NBC's Tonight Show was becoming one of the most popular late-night TV shows.
With a steep decline in weekly theatre attendance, studios were forced to find creative ways to make money from television - converted Hollywood studios were beginning to produce more hours of film for TV than for feature films. [In mid-decade, the average film budget was less than one million dollars.] And by mid-decade, the major studios began selling to television their film rights to their pre-1948 films for broadcast and viewing. The first feature film to be broadcast on US television (on November 3, 1956), during prime-time, was The Wizard of Oz (1939).
http://www.filmsite.org/50sintro.html
