The holiday travel season is coming, and here's some important new data that might help you enjoy it more: The cheapest airline fares can be found 57 days before your planned departure. What's more, the best day of the week to buy those tickets is now Sunday—which has changed in recent years. It used to be Tuesday.

The new information is thanks to a detailed report by the ticket processing center at Airlines Reporting Corporation (ARC), a service used by travel agencies that represents half of all airline tickets sold in the U.S. ARC looked at a 19-month period of ticket sales (almost 130 million tickets) and saw that the lowest average prices were found on Sunday. Second runner-up? Saturday.

This chart by ARC shows fares purchased on Sunday can be an average of $65 cheaper

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Frequent fliers who consider themselves well-versed in fare fluctuations might be surprised that weekend-purchased tickets are now cheaper. For years, travel sites have warned travelers to avoid booking on weekends and drilled into our heads that Tuesday is the best day of the week to look for airline deals. What changed?

Two major cultural factors can contribute to the shift, according to a story in the Wall Street Journal. First, think about how the airline industry used to work: Execs would come in on Monday, look at how sales progressed over the past week, and make decisions about which fares to discount for Tuesday (which historically were advertised in newspapers). That's no longer the case due to technological changes that altered the way people shop: Social media can help disseminate deals at pretty much any time, and mobile devices mean people can buy from the couch, so airlines are hoping to snag some of these potential passengers noodling around online.

Airlines still send out deals on Tuesday (usually via email), but the people who are buying airline tickets during the week are likely to be business travelers, who likely will already be paying steeper fares due to their scheduling needs. So airlines don't really need to discount in order to get them to buy tickets.

This graph by ARC shows the fluctuations in fares and how the lowest average price can be found 57 days before departure

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Besides knowing what day of the week to buy, a key part of shopping for the best deals is knowing how far ahead to purchase. According to ARC's report, most Americans don't purchase far enough in advance to get the best price on domestic airfares: Most of us buy about 30 days ahead. The sweet spot is actually 57 days before your planned departure, when fares are an average of 19 percent lower. This is actually more lead time than travel sites have told us in recent years: In 2012, the magic number was 42 days out. (For international, three months out is better.)

So if you're planning to fly somewhere for Christmas, that would mean you should book your trip sometime right around... today. But here's my question: If the 57-day window doesn't fall anywhere near a Sunday, do you pull the trigger before day 57 or wait for the next Sunday to roll around? The answer might be to split the difference and buy on the third least-expensive day of the week. Yep, that would be Tuesday. [ARC, WSJ]

Photo by Getty Images North America


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