This story originally appeared on Travelandleisure.com.

CheapAir.com announced the results of its annual airfare study on Wednesday, analyzing 917 million fares from nearly 3 million trips to determine when the best deals are typically available for domestic flights in the U.S.

Based on their data, the best time to buy is roughly 70 days before departure. It’s a pretty significant increase from last year's study, when the site reported most inexpensive fares could be found 54 days in advance.

“Our Airfare Study aims to explain the ‘method to the madness’ and give travelers a sense of the best time to buy and fly,” CEO Jeff Klee said in a statement. “We hear from travelers all the time who are frustrated that a fare they saw one day is dramatically higher the next.”

Klee added that it’s “equally disturbing” when airfare plummets just a few weeks after you book a trip.

The Prime Booking Window

On average, fares will change 62 times over the course of 11 months, with each dip and rise changing the price by approximately $36, according to the study.

CheapAir.com recommends that travelers closely monitor airfares during the so-called “Prime Booking Window,” which is from four months to three weeks (or 121 to 21 days) prior to the departure date. During this time, travelers should closely monitor their travel route, and prepare to pounce on an attractive price.

“Fares bounce around, but frequently are available for within 5 percent of the lowest price,” the study claims.

Last-minute Trips

CheapAir.com added that travelers are taking a risk if they try to book a flight 20 to 14 days out. “If you’re lucky, you might still find a good deal,” according to the study, “but in general, fairs rise, and flights sell out.”

After that, things get more hairy. Between 13 and 7 days prior to departure, travelers can expect to pay $85 more than those who booked during the Prime Booking Window — and “deals are a long shot.” And for seriously last-minute fliers, airfare can skyrocket by $208 in the final six days leading up to a trip.

The Best Day of the Week to Book

CheapAir.com's study also rejects the commonly held myth that certain days of the week are better for flight shopping. “The average fares purchased each day were virtually identical,” the study concluded. (If someone tells you to buy on Sunday or Tuesday, they're wrong.)

When to Book Seasonal Trips

But the time of year can have a significant factor. For summer trips, the best fares were booked 47 days in advance, while fall and winter trips should be booked 69 to 62 days in advance, respectively. For spring vacations, deals were typically scored 90 days ahead of departure.

To further help travelers pinpoint the very best time to book flights for their specific trips, CheapAir.com developed a “When to Buy Flights” tool that shows the lowest historical ticket prices, the best day to book, and the broader “Prime Booking Window.”

Travelers can also keep a close eye on routes by setting an airfare alert through a search engine, like Google Flights or Kayak, or with a travel app, like Hopper, which will send an alert if flight prices on your desired route plunge.