My friend Jeddy tells me that TV stations operate on a 13-week schedule, which is why all the TV seasons have 13 or 26 episodes. For example, with the Simpsons, they air a new episode, once a week, for 26 weeks. Then for the next 26 weeks, they repeat the episodes. That makes a full year. I guess that's a successful formula, since they've been doing it for decades now.
All the TV shows I watch or know about are weird, and they don't follow this typical 13-week schedule. Spongebob Squarepants apparently is done in 20-episode blocks, Melissa & Joey get 15-episode blocks, and I don't even know about The Fairly Oddparents anymore, because it took them three years to show all the episodes from last season.
Speaking of 13 weeks, Little Orphan Annie is the only comic strip I know of that was drawn in 13-week chunks. Harold Gray didn't want to draw a comic strip every day; he preferred to rush out 91 strips in a three-week period, then take ten weeks off. Or something like that; I don't know how long of a break he took between drawing sessions.
If you're a careful reader, you can tell when the break between 13-week segments occurs, because his handwriting will change completely, or the characters will look different. That, and he'll completely change the plotline by having Little Orphan Annie move somewhere else. It's kind of funny to see the comic strip change so much, in a short period of time.