Up until three years ago, adding a Saturday-night stay to your itinerary would dramatically cut your airfare. But in 2005, in a much publicized move, Delta simplified its fares. It stopped charging extra for advance-purchase tickets that return on weekdays instead of weekends. Other airlines copied the move. The low fares you saw advertised on the Web didn't have Saturday-night stay restrictions, as a general rule.

But that changed recently. United, Delta, and Continental have recently brought back to life requirements that you must stay on a Saturday-night. The rule isn't applied to all tickets, but it is being applied to tickets on many popular routes, such as between New York and Houston. American and Northwest have also rolled out the policy on a more limited set of routes. The news was first reported by TheStreet.com.

Our advice: Once again, advertised fares may come with a Saturday-night stay catch. Be flexible in picking your travel dates, whenever possible, because you may save hundreds of dollars.