Sweeps

I can't say his full name or he would feast on the blood of the innocent, and I’m saving that for sweeps

—Stephen Colbert

The periods when advertising rates are set, based on the Ratings received by the programs shown at that time. Every network pulls out its big guns to get as large an audience as possible for its shows, so as to maximize its revenues for the coming quarter.

Happens four times a year (February, May, July and November), but you always know when they're here, because incredibly obvious programming stunts abound—news programs start running features like "Group Sex In Our Community" and Special Guests pop up on every Sitcom. Local news programs are probably the most notorious for this; locally-made programming makes far more money for the station (since they don't have to share with the network), and these days, news programs are usually the only local content a station has, so the ad blitz goes there.

The rec.arts.tv FAQ explains sweeps in more detail.

It's rare to see a rerun during sweeps. It's rare to see a new episode in any other month. Almost all reality series will try to have either their premieres or finales during these periods, often both through straddling two periods, if possible.

Examples of common Ratings Stunts during sweeps include: