ATLANTA – Excitement about a trip to Pittsburgh for your brother-in-law’s wedding that you booked months in advance suddenly turns to gloom as the telephone rings a couple of hours before your flight.

On the other end of the automated call from the airline, a computerized voice tells you that your flight has been canceled and you’ll have to wait until the next day to catch another flight on that carrier.

What now?

“Point No. 1, you as a traveler cannot predict when that might happen,” said Ed Perkins, a contributing editor to smartertravel.com. “Point No. 2 is when it happens, if it happens, your first decision is, ‘How critical is it that I get to my destination today?’ If it is very critical, you go to your airline and look at their contract of carriage and see what they will do in the event of a cancellation.”

When you buy a ticket on an airline you enter into a contract with that airline. This contract of carriage spells out the obligations and rights of a carrier and a passenger.

Some of the terms are set by the airline, while others are standard terms or terms provided by applicable law. Contracts of carriage can often be found on an airline’s Web site.

Here’s a tip sheet to help you understand your options if your airline changes the time of your flight or cancels it.

There are no guarantees. Continental Airlines advises passengers that its schedules are subject to change without notice and that times shown on its tickets are not guaranteed.

Request a refund. Most airlines will refund the amount you paid for a ticket if they cancel your flight and can’t accommodate you on another flight that gets you to your destination on the day you were expecting. Most will not compensate you, however, for money you lost because your flight was canceled – such as missing a client meeting and not getting a big account.

Try to be rebooked. Some major airlines have interline agreements that allow them to easily rebook a passenger on another carrier.

For example, under certain circumstances United Airlines might arrange for transportation on another carrier or a combination of carriers if it is unable to provide a new flight that is acceptable to the customer. In that case, the passenger would be entitled to the same class of service as the original flight at no additional cost.

Ask for a hotel room. American Airlines says that if a delay or cancellation was caused by events within its control and it does not get a passenger to his or her final destination on the expected arrival day, it will provide “reasonable” overnight accommodations, subject to availability.

Provide Contact information. Make sure your airline has your e-mail address or telephone number so it can alert you in the event that it cancels or reschedules your flight. There’s no hard and fast rule saying how much advance notice an airline must give you about a flight cancellation or schedule change. The last thing you want is to find out there’s a problem when you get to the airport.

Delta Air Lines encourages its passengers to sign up for a service that sends them voice, text or e-mail alerts about changes to their flight.

Try to negotiate. With fewer people flying, airlines are eager to generate loyalty and keep customers coming back. You might be able to use that to your advantage if an airline inconveniences you by canceling or rescheduling your flight.