No they're not that simple anymore.
You've got the regular TV season that starts in September and ends in May; then you've got "mid-season" replacements that start anywhere between January and April and usually run through the summer; THEN you've got summer replacements that usually start May or June and run up to September...
Then, you've got all these "sweeps" months where big events are supposed to happen, thus, TV shows have to get exciting or interesting to bring the viewers in, and apparently those are like every quarter or something.