Time for another TV Question and this one is one of those that happens every year. In fact this one happens four times a year. It’s the often misunderstood “Sweeps” periods.
There are four “sweeps” periods during the year (though the fourth sweeps period in the summer is less important): November sweeps, February sweeps, May sweeps and August sweeps. It is during these “sweeps” periods (usually lasting 3-4 weeks) that the advertising rates for a network’s schedule of shows (and for local channels’ programming) are set with sponsors and advertisers.
It is of the upmost importance that a particular network get the highest ratings possible during these sweeps, so that they can charge higher rates from advertisers. Further, a secondary goal for the networks is to “win” the sweeps period both in terms of overall ratings and in terms of the ratings for “favorable demographics (e.g. people 18-49, especially those 18-39 years old), again with the eye on charging higher advertising rates (and thus generating higher profits, which in turn can lead to the development of better shows, etc.). So sweeps periods are often when networks roll out “big” specials (such as miniseries, TV movies or “reunion” shows) and the “best” (i.e. controversial) episodes of some of their most popular shows.
Of these four sweeps periods, the May sweeps is by far the most important, for two reasons. Firstly, May is most usually when shows have their season-closing episodes. Season-closing episodes (e.g. cliff-hangers) are important in helping to draw viewers back to those same shows when they premiere in the fall. Secondly (and more importantly for the network overall), the May sweeps are the ones used to set advertising rates for the upcoming fall premiere season.
November sweeps set the ad rates for the following spring; they are probably the second most important sweeps period as they are used to gauge the success of new show that premiere that fall. February sweeps set the ad rates for the following summer. The least important, fourth sweeps period, is in August, and seems to be used to set ad rates for fall.