The findings of the Annual Airfare Study, which crunched 921 million airfares from 2.9 million trips to find the best and worst times to buy an airline ticket, were released today.

For the second consecutive year, the study found that 54 days out is, on average, when travelers can get the best deals on domestic flights. However, the best timing depends on when and where passengers fly.

The study found that the lowest fare for a given flight changes an average of 71 times between the time it’s announced and the day the plane takes off. In other words, the price of a flight changes on average every 4.5 days, and each change averages $33 up or down.

“People always ask why air fares have to be so crazy complicated,” said Jeff Klee, CEO of CheapAir.com. “We try to share as much data and insight as we can to make the process a little less daunting.”

Klee added, “The most important rule is fairly obvious: don’t wait until the last minute, as that rarely works out. But beyond that, you also want to be careful not to buy too early. I always suggest that travelers check fares early and often and get familiar with the market. Then, when you see a good deal pop up, grab it, because it likely won’t last very long.”

The Airfare Study identifies five booking windows in which travelers buy flights which CheapAir has labeled:

“First Dibs” approximately 6 – 11 months in advance, when flights first open for sale and fares tend to be on the high side.

“Peace of Mind” 3½ – 6 months in advance, when fares are at a modest premium but options abound.

“Prime Booking Window” 3 weeks – 3½ months in advance, when airfares are the cheapest, on average. This is typically the best time to buy airline tickets.

“Push Your Luck” 2 – 3 weeks in advance, fares can vary dramatically but are often rising significantly, particularly as flights fill to popular destinations.

“Hail Mary” 0 – 2 weeks in advance, this is when airfares are highest, on average $150 more than booking in CheapAir.com’s “Prime Booking Window.”

The study also examined whether the day of the week you click “purchase” makes a difference. The study found that there is on average less than a 0.6% (less than $2) difference between purchase days of week. However, the day of the actual flight is a different matter. Tuesday and Wednesday are the cheapest days to fly, and Sunday is the most expensive, with an average difference of $73.