Layout of a Season

Many shows have some form of structure to their seasons, rather than just being collections of stories. This article mostly assumes that the series in question are US full-length series of around 20-25 episodes with some to low Season Fluidity. Even in cases of British Brevity, the season opening and closing episodes will tend to be similar in style to those in the US, regardless of the season's length.

Season Opener
This will often be big and spectacular, to lure viewers back to the show. It is not uncommon for new characters to be introduced to the show or for major plot changes to take place. See also Premiere and Pilot. It is also one of the most likely places to find a double length episode (and even when the first two episodes are both normal length, it's not at all uncommon for networks to show them back to back).

Lucky Episode 13
Some series only have the first 13 episodes of the first season commissioned, with the promise of more if it is successful. For this reason, the writers will work out a 13-episode arc plot so that if the series is not renewed, it will still have a satisfying ending. If it is renewed then they need only rewrite the final script or so to introduce another threat or more plot threads.

The Ep-15 Shift
Episodes 15-16 of a season tend to fall around the time of Sweeps, so networks will have big spectacular episodes to attract viewers. Plot shifts are not uncommon here.

Season Finale (Ep-20 to Ep-25)
Again, big and spectacular, usually tying up all of that season's emotional and plot arcs, although an end of season Cliff Hanger is possible. There may well be character departures by various different means. In cases where the season finale is made prior to the decision on whether or not to renew the show being made, expect a finale that ties off most major plot threads, but leaves enough dangling to make it easy to carry on if the show is renewed.

If this is a two-season drama - even if the seasons are of irregular length - prepare to be burned.

Season Openers
 * Horatio Caine's trip to Brazil in the Season 5 opener of CSI: Miami.
 * Buffy and the gang starting college in the Season 4 opener of Buffy the Vampire Slayer.

Lucky Episode 13
 * Episode 13 of the first season of 24 ends with the team managing to kill the assassin who has been threatening Senator Palmer's life - but there is also a minor cliffhanger, as a second assassin is dispatched. Had the season been cancelled after 13 episodes, this last scene could have been easily cut to end it "properly".
 * The 13th episode of the first season of Glee was this trope to a tee.

The Ep-15 Shift
 * "Phase One" of Alias is a textbook example.
 * The nuclear explosion in season two of 24.
 * My-HiME ended the Searrs arc in episode 15 (in a Your Princess Is in Another Castle moment), and episode 16 introduced a new and unexpected plot twist.
 * "Surprise" and "Innocence" on Buffy the Vampire Slayer.
 * "The Coming of Shadows", a second season episode of Babylon 5, did this well enough to win a Hugo Award. (It was actually episode 9, not 15.)
 * Each of Castle's three full seasons has had a major two-parter with a big guest star representing a federal agency right about this time. They were:
 * Season 2's Tick, Tick, Tick... & Boom!, guest starring Dana Delany, playing a specialist from the FBI.
 * Season 3's Setup & Countdown, guest starring Adrian Pasdar, playing a member of the Department of Homeland Security.
 * Season 4's Pandora & Linchpin, guest starring Jennifer Beals, playing a member of the CIA.

Season Finale (Ep-20 to Ep-25)
 * Doctor Who: "Bad Wolf/The Parting of the Ways", which ended with the regeneration of the Ninth Doctor into the Tenth Doctor and with Captain Jack Harkness being left behind on the space station.
 * House MD: Season 6 begins with  This is not referenced again until a couple of episodes before the finale.
 * Terriers is a textbook example of a season finale that works as either a cliffhanger for next season or a satisfying resolution to the series. (It turned out to be the latter when the show was not renewed.)
 * The House Of Elliott, a three-season Period Drama with ten episodes per series and no finite plot, more similar to an American series than the typical British literary adaptation or original period miniseries, ended on a cliffhanger with one of the sisters threatening to walk out on the eponymous fashion house. However, the BBC then decided not to commission another series, so the series was left dangling.