The least expensive airfares processed by ARC in 2011 were purchased six weeks in advance of flight departure.

According to ARC, the average fare for such tickets was 5.8 percent lower than the overall average fare of $358.30, the largest discount for any point during any given 120-day advance purchase period last year. "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," according to ARC managing director of data and analytics Chuck Thackston. "It is just what the data indicates and we have seen this pattern over the last four years."