Announcer: The start of television's second most exciting season - midseason - is just two hundred exciting seconds away!
—The Simpsons, "Homer to the Max"
If a show's really, really lucky, it'll last a number of seasons. Some shows are lucky to get one. Meanwhile, some of the new programming premiering at the beginning of a season flops so badly that it's canceled before even being allowed to complete a full run.
This leaves the network with an uncomfortable situation - how to fill out the rest of the season. Often, networks will throw up reruns of popular shows in the failed show's timeslot, or fill the void with Made For TV Movies or other special programming. The usual go-to guy for such a situation, however, is the Midseason Replacement.
The Midseason Replacement is typically a show network execs don't consider strong enough to premiere at the beginning of a season, but is held in reserve for a premiere later when time slots are needed to be filled after another show has gone belly-up. Since the midseason replacement is considered to be a weaker show than what the network was originally hoping for, expectations often aren't very high. Another reason a show might be a midseason replacement is because of production problems - The Simpsons is one such notable example, as is Star Trek: Voyager which simply had to wait for the rest of the network to premiere. Also, due to the need to stretch out episodes if a show airs over the entire TV season (September through May), a show may premiere midseason simply so they can air all the episodes with few or any breaks (this is especially common for serialized programming or reality shows).
Midseason replacements premiere, as the trope name suggests, in mid-season, which is typically anywhere from late December to early May. Any date later than that, and it's considered a late season replacement, which can premiere from late May into June. Late season replacements are usually considered by networks to be very weak shows, may have only a very limited number of episodes ordered, and are considered to have almost no chance of actually being picked up. Cancellations before the show even premieres are not rare. A late season replacement may result from a network wishing to fill gaps in its summer programming, to provide an alternative to summer rerun schedules, or may have been originally slated as a potential midseason replacement but in which feared gaps in midseason programming never materialized.
Note that this trope only specifically applies to broadcast network programming in the United States; different countries have different conventions, and the practices of cable networks in the United States may vary just as greatly. For example, USA Network's Monk, generally considered to be one of the strongest shows on cable, premiers new episodes in the middle summer months and winter months, in a unique (though common practice on USA Network) season split-up.
- The Simpsons, Law & Order, and Seinfeld, all of which were midseason premieres, and all of which became highly successful shows. Seinfeld in particular was threatened with cancellation at the conclusion of its initial season, a sparse 5 episodes, but was given a second chance, and the rest is history. The Simpsons was meant to be a fall premiere but several early episodes had to be re-animated.
- Buffy the Vampire Slayer. Its first season was only twelve episodes. Sarah Michelle Gellar even commented that when she told people what she was working on, they said she would get something better next season.
- Family Guy - having been canceled and then Uncancelled is notable enough, but its "uncancellation premiere" almost qualifies as a late season replacement having originally premiered the first week of May 2005. In a rare case of tactful thinking, FOX executives were confident enough of the show's popularity to bring in huge ratings despite the late premiere date.
- American Dad likewise followed Family Guy for its premiere on the first week of May 2005.
- Reality Shows very frequently have midseason or late season premieres as emergency replacements for programming that has failed unexpectedly. Networks have pretty much refined the quick formulation, production and execution of reality shows (often in a matter of weeks from conception to premiere at most) to a fine art at this point.
- Most prime-time Animated Series are intentionally midseason replacements, the reasoning being that they're more expensive and time-consuming to produce and the networks don't want to have it all go to waste if the show bombs. So the first season is intentionally made shorter as a sort of trial run.
- Star Trek: Voyager premiered in January, launching the UPN network. Star Trek: Deep Space Nine also began its run in January. Both had shortened first seasons (albeit only marginally so for Deep Space Nine), and both did well enough to last seven seasons.
- Toward the Terra is an anime example. After Ayakashi Ayashi flopped so badly that was canned midways, Terra was allowed to fill the gap, occupying TBS's anime primetime slot that previously housed "A grade" shows like Gundam Seed, Fullmetal Alchemist and Blood+.
- The Office was not only a midseason premiere, it almost didn't last to a second season.
- Quantum Leap.
- The A-Team premiered in midseason in 1983 and ran successfully for five seasons before Cancellation. It still has a Cult Classic fanbase and recently spawned a Movie some 27 years after its premiere.
- Grey's Anatomy was initially one of these.
- As was Castle.
- 24's first three seasons all started in September and ran until May, but complaints about a show in real time being stretched out over the entire season caused them change to a "non-stop season" format, in which every week had at least one new episode; for obvious reasons, for the finale to hapen in May, the show had premiere midseason.