To book or not to book, that is the question. Knowing the best time to book a flight can be an agonizing decision, but a new study offers some much-needed advice. According to Airlines Reporting Corp. (ARC), an Arlington, Virginia-based company that handles ticketing transactions between the nation's airlines and travel agents, passengers can get the lowest airfares if they buy their ticket six weeks before the flight.

ARC came to this conclusion after surveying over $80 billion ticket sales across 144 million transactions from Jan. 1 to Dec. 31, 2011 for flights with a U.S. origin and destination. The company found that those who purchased tickets six weeks in advance paid, on average, nearly six percent less for airfare.

Many people have long thought that the further out from your flight that you buy an airline ticket, the less expensive it will be. Our data indicates that this isn't necessarily true, said Chuck Thackston, managing director of data and analytics at ARC. Although low fares are available over the 120-day cycles in 2011 that we analyzed, the vast majority of tickets costing below the average fare were purchased about six weeks before the flight date.

We're not advising people to purchase tickets only at this time during the cycle as there is no guarantee they will receive the lowest price of the year, Thackston cautioned. It is just what the data indicates and we have seen this pattern over the last four years.

The study also showed that ticket prices rise dramatically about a week before the travel date and can rise nearly 40 percent above the average price if a passenger purchases the ticket on the day of the flight.