For the second year in a row, has found that the best time, on average, to purchase a plane ticket for domestic U.S. flights is 54 days in advance. 

To assemble its 2017 Airfare Study, CheapAir crunched 921 million airfares. On average, the website found, the price for a given trip changed 71 times, and with each change the lowest prices rose or fell by $33. 

"So, clearly, it matters a huge deal when you choose to pull the trigger on your flight purchase," CheapAir said in a summary of the results.

Though 54 days in advance works out to be, on average, the cheapest time to book airfare, the actual cheapest time varies widely on a ticket-for-ticket basis, CheapAir said. As such, the company designated what it calls its "prime booking window," which ranges from 105 days to 21 days in advance of a flight.

"This zone is where the cheap airfare hunter wants to concentrate his or her best bargain-shopping efforts," CheapAir said. "Keep in mind that fares will still fluctuate within this rather large window, but in general, the sweet spot is in this zone – where the best fare tends to pop up and fares tend to be within 5% of their lowest."

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