Travel booking platform Qtrip.com crunched more than 917 million airfares in 8,000 markets to reveal the best and worst times to book US domestic flights for its Annual Airfare Study. The study finds that airfares have become less volatile over the last few years, and the best deals appear progressively earlier.

Data from six consecutive years of the Annual Airfare Study show that the best time to buy flights for domestic US trips has changed significantly. On average, the least expensive airfares were available 77 days before departure in 2019. That’s up from 76 days in 2018, 70 in 2017, and 54 days between 2013 and 2015. In 2019, airfares changed an average of 54 times by USD 34 each swing. This is down from 61 changes in 2018 and 62 in 2017.

Buying airline tickets between 21 and 127 days before departure is ideal. Qtrip calls that the Prime Booking Window, when fares are on average are within 5% of their low point. The day you fly is much more important than the day of the week you buy. On average, a flight travelling on Tuesday or Wednesday is USD100 less than the same itinerary on Sunday, which is by far the most expensive day to travel. However, contrary to some popular myths, the day of the week that you go online to buy your tickets still doesn’t matter.

Between the day a flight is published (about 11 months out) and take-off, airfares pass through a number of Booking Zones, each of which has opportunities and risks:

First Dibs: 315 to 202 days out. Spend USD 50 more but enjoy the most seat and itinerary options.

Peace of Mind: 201 to 128 days out. For USD 30 extra, get balance between options and price.

Prime Booking Window: 127 to 21 days out. Book here to land within 5% of the lowest price.

Push Your Luck: 20 to 14 days out. You might score a deal. But probably not!

Playing with Fire: 13 to 7 days out. Choice even more limited and prices continue to rise.

Hail Mary: 0 to 6 days out. Usually reserved for emergencies, these fares cost USD 200 extra.

The Prime Booking Window does change seasonally. The best fares appear 85 days out in spring, 113 days out in summer, 77 days out in fall, and just 49 days out in winter. Excluding the December holidays, winter offers the best deals. Summer will cost USD 25 extra on average.

Bottom line, book early to get the most options but wait for the Prime Booking Window to get the best deals.