No, they do, though it’s not as rigid as it used to be. Generally speaking, networks take the holidays off, which is generally from mid-December to mid-January. That’s when schools are on break and kids are more likely to be out on the town or going to see movies instead of watching tv at home.
They also take a few weeks off in Spring, presumably around the time of Spring Break when all the kids head down to Florida and Mexico to get wasted.
And then there’s Summer… oh Summer, how you used to be totally sucky and devoid of ANYTHING to watch. At least it’s gotten somewhat better now, with some good cable shows running half their seasons in the summer.