[evay]

There are three periods (November, March, and May) during the traditional TV season (Sept-June) when networks measure ratings to determine what prices they'll set for advertising spots for the following year. In order to jack up ratings as high as possible, they'll run event programming, do stunt casting (Spiner on ENT, for example), or have big splashy season finales.

of course, since it's not like it's a surprise that those months are the ones being measured, it rather defeats the purpose, and the nets are kind of shifting towards year-round programming anyway to compete with cable, the Sweeps (and Nielsen ratings, for that matter) really need to be ditched.