If you’re starting to plan your summer vacation, you may think buying your plane tickets months ahead of time is a cost-saving move. But according to a new study, that may not necessarily be true.

After looking at millions of airline ticket transactions in the past four years, Airlines Reporting Corp., which acts as a middle man on ticketing transactions between airlines and travel agents, found that fares were generally at their lowest point about six weeks before a flight.

Chuck Thackston, managing director of data and analytics at ARC, said at about 100 days out, airlines offer deals to stimulate demand. But as those deals expire and seats are purchased, prices tend to bottom out right around 42 days before a flight.

In 2011, tickets purchased six weeks in advance were almost six percent lower than the year’s average fare of $358.30, mirroring a trend seen in years past — in 2009, tickets bought about seven weeks before a flight were 26 percent lower, and in 2010, tickets were almost 20 percent cheaper six weeks out.

Of course these things are never an exact science, but when in doubt, experts say to shoot for that six-week timeline.

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