What are "sweeps"?
Each ratings "sweep" builds a fuller, more complete picture of how well shows are performing in America, from the biggest urban area through to the tiniest TV market. Outside these periods, most ratings data is gathered using automated "set meters" (measuring exactly when TVs are on and what they're tuned to) and "people meters" (like set meters, but targeted so that more granular demographic data is available) that transmit data on what people are watching directly back to Nielsen Media Research for fast turnaround. During sweeps, viewer diaries play a big role too.

Why's it called a "sweep"?
Nielsen used to mail out the diaries across the nation, "sweeping" from east to west. Advancements in postal delivery now render this unnecessary, but the name has stuck.

Any other reason why it's so important?
Nice, extended runs of original episodes of network TV shows. Re-runs don't tend to rate as well as new episodes, and the networks like to put on a strong showing.

The schedule: 2006
November 2 - November 30: 209 markets measured

The schedule: 2007
February 1 - February 28: 210 markets measured
April 26 - May 23: 210 markets measured
July 5 - August 1: 207 markets measured
November 1 - November 28: 210 markets measured