How Far in Advance Should I Book Flights?

Next to the tax code, there may be nothing more confusing than airfare. Dynamic pricing, capacity-controlled rate tiers, seasonal shifts—how is anyone supposed to know the best time to buy flights? Frommer's has the answers. The following rules of thumb and price-shopping tools will guide you to the absolute best day to purchase your flights.

The experts are forever crunching data in an attempt to predict the ideal booking window. It's a moving target because the airlines' pricing formulas are always changing. But generally speaking, these advance-purchase deadlines will serve as a decent, if rough, rule of thumb. Last-minute airfare deals pretty much don't exist anymore. It's ideal to search for airfare at these moments:

• International airfare: 6 weeks ahead

• Domestic airfare in summer: 47 days ahead

• Domestic airfare in autumn and winter: 62–69 days ahead

• Domestic airfare in spring: 90 days ahead

But that's not the end the story! Now we're going to introduce you to a bunch of additional tools. Using the lesser-known secrets in the upcoming slides, you can put an even finer point on the exact day that the flight you need will cost the least. 

Prices inevitably zoom upward when there are major events going on in your destination—marathons, celebrations (like this hot air balloon festival, pictured), big conventions, holidays, and so on. Before you begin looking for flights, do some rudimentary research to make sure you're not planning to go at the wrong time, when both airfare and hotels will cost way more than usual. Consult one of the Frommer's guides' "When to Go" sections, look for events calendars on your destination's tourism bureau website, and find out the major school holidays where you're going. Once you know the dates not to fly, it's time to zero in on the best dates you should fly. 

Download the free Hopper app. When you search flights there, Hopper will look at the pricing trends across the entire calendar to give you a sense of when your flight will cost the least, when the ideal booking deadline will be, and how much you'll save by choosing a different day. Lots of websites have features that may tell you vaguely when to book, but Hopper uses its data to get specific about predicting the right moment to pounce. 

It's possible to book via this app, but don't do that yet—armed with this information, you should check a few more tools to make sure you're saving the most money. 

Momondo is a website that trawls many different airfare booking sites and shows you the best results. That's useful by itself, but the site also automatically includes a rare resource: a graph that shows you whether the prices Momondo found are lower or higher on the dates surrounding the dates you plugged in—those are the blue vertical bars with the dates going across the screen. These results help you refine your ideal travel window. But there are ways to get even more exact price quotes from the airlines themselves, and that's the next step.

Once you know how far in advance to make your purchase, you can now narrow the options even more. At that right moment, search using the airlines' websites. Some airlines (Delta, Norwegian, but too few others, honestly) help you find the cheapest dates by offering "flexible date" search options—always make sure to use this feature to find the best deal, even if it means shifting your travel dates slightly. Some airlines even display full calendars of price quotes so that you can choose the lowest date. Southwest's calendar (pictured) and Alaska's calendar are two of the easiest to use.

Another super-useful addition to your flight-booking toolbelt is Skyscanner's whole-month calendar pricing. Like Momondo, Skyscanner is an aggregator, which means it doesn't sell flights directly, but scans lots of booking engine sites and collects the prices on one page for you. That's how the site can get an instant sense of the cheapest month to go, and it sorts the results with that as a priority. It's another great way to get a bead on the best time to fly—and it's a big reason Skyscanner landed near the top of our "Best Airfare Booking Sites" awards list.

Having gone through these steps, you can feel confident that you've zeroed in on the very lowest airfares that fit your travel plans. You're now ready to book. Check out our list of the 10 Best (and Worst) Airfare Booking Sites for a headstart on doing the actual deed.