The following information is from Which Price is Right?, Februrary 2003

And that's just the beginning. All of those prices need to be managed, all the time. All of the major airlines ( except Southwest ) participate in a joint fare-publishing enterprise called ATPCO. ATPCO collects fares and rules ( such as advance-purchase requirements, refundability, and so on ) from each airline. ATPCO then publishes those fares back out to the airlines and to the reservation services. In the 1980s, fare changes were filed once a day. If someone changed fares, you found out on Monday and filed your competitive response on Tuesday. ATPCO was closed on weekends. Now ATPCO accepts fare changes three times every weekday -- at 9 AM, 11:30 AM, and 7 PM Central Time -- and once each on Saturday and Sunday. From 5 fare changes a week to 17.

As much as it can look like a kind of silly game -- why not just take a pass on a couple of those changes? -- it's not. "If a sale comes in at 7 PM from a competitor," says Continental's senior director of pricing, Bob Lancaster, "and you miss the 9 AM filing the next morning -- you don't get your changes in until 11:30 -- you're in trouble." So when Continental has 7 million prices in the market, those prices can change not just every day, but several times a day. Typically, says Lancaster, the airlines collectively change 75,000 prices a day. On the morning after someone files a 7 PM sale, there might be 400,000 price changes across the markets.

Did you ever wonder whether it matters what day of the week or what time of day you book airfare?

The co-founder of Lessno.com, a Web site that offers international flights, says it does matter. "Our experience with airlines has shown us that they update available seats around 12 a.m. Eastern time every day," said Assen Vassilev, Lessno's CEO. He added that weekend specials are usually released Thursday night. So, Vassilev said, late Thursday night is a good time to check for deals.