The
kiss of death for an
American network TV series is to be aired as a summer replacement, debuting during the months when most time slots are filled with
reruns and most viewers are outside enjoying the good
weather.
Unlike a midseason replacement, the summer shows are assumed to have no chance.
Summer is the dumping ground for shows that were paid for but just came out badly, or didn't score with the focus group. The network will cut its losses by terminating the show but getting some commercial revenues out of it for a few weeks. They're filler.
Sometimes, summer replacements are shows that have fallen out of favor -- maybe the executive that OK'd the deal got fired, for example, and no one else at the network believes in the show. By dumping the show into a summer time slot, executives can get their money out of the show without having to risk it becoming a hit (and thus having to admit they'd made a wrong decision; TV executives are like kindergarteners).
Things were different in the 1970s when variety shows were all the rage. Networks would use the summer to test out new variety shows, to see if a host could sustain an audience during several shows. The Sonny and Cher Comedy Hour was one such, debuting as a summer replacement for The Ed Sullivan Show.
Very, very rarely do summer replacements become bona fide members of a network's prime time lineup, let alone a hit. But like a lot of rules, this one's got a big, big exception: Seinfeld.
For more commentary, there's a good TV column in the Seattle Post-Intelligencer from June 2001: http://seattlep-i.nwsource.com/tv/27709_tv18.shtml. Discusses the particular cases of Go Fish and You Don't Know Jack.