We came across a website that analyzed prices for over four million flights in 2013. They found the best time to book a domestic flight in the U.S. is 54 days before you plan to depart.  If you booked any sooner or later, you paid a much higher fee.

If you plan to travel internationally, like maybe a trip to Mexico, they say book 89 days in advance, 101 days for the Caribbean, 129 days for Asia and 151 days if you're planning a European vacation like the Griswolds.

FYI, The only time the 54-day time line doesn't apply is for Thanksgiving and Christmas flights.  They say your best off booking those flights six months prior on June 4th.