From 1955 to 1975, Gunsmoke was the longest-running primetime drama on US television. With 20 seasons, there were 635 episodes. The premise of Gunsmoke was similar to most western movies. Marshal Matt Dillon protected the wild west town of Dodge City against outlaws and corrupters. James Arness, who played Dillon, appeared in every episode.
According to imdb.com, “[Gunsmoke was] slated to be canceled in 1967 due to low ratings, but then-CBS president William Paley reversed the decision. He moved the show from Saturdays to Mondays (cancelling Gilligan’s Island in the process), placing it back in the Nielsen’s Top Ten (Paley and his wife were both big fans of the show).”
Steinberg, Cobbett. TV Facts. New York: Facts on File, 1985, 76.