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 (February) Sweeps, so networks will have big spectacular episodes to attract viewers. Plot shifts are not uncommon here, as these are the early installments of the "Back 9" extension. 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.
For examples, see Front 13, Back 9 The Ep-15 Shift