We have the same sort of thing over here.
There are basically 4 "seasons" on British TV, 12 weeks each.
You will note for example that weekly series all tend to stop just before Xmas (Torchwood for example).
Then a new season of shows starts in the new year.
Also at the end of the summer holiday, and start of Autumn, another new season starts.
If you buy one of the TV listings magazines they often talk about the "new season" of programs at certain times of the year.
I also think that American TV tends to milk their shows.
Over here we often have series that last 6 weeks. You can fit 2 series in a 12 week season.
But US series just seem to go on and on and on. I think they try to keep them running to maximize the advertiaing.
If you compare fawlty towers, where we only ever made 12 episodes, to Friends, where they made about 200.