I saw it this morning, but I have seen it other times as well that I can't quote.

(“NBC Nightly News With Brian Williams” narrowly overtook “World News With Charles Gibson” on ABC to become the most-watched of the three network evening newscasts in November, a so-called sweeps month regarded by advertisers and the networks as a bellwether of the broadcasts’ fortunes.)

I have heard of sweeps and have seen them widely labled WITHOUT "so-called" being attached. But seeing it this morning made me really wonder when it should or should not be used.