People who have flexible dates get the best fares. There's no magic day to buy a fare, although there are cheaper days to fly--for domestic U.S. travel it's Tuesday and Wednesday, and for international trips it's usually Monday to Wednesday or Thursday.
You can often save hundreds of dollars by flying when the going is cheap, and sometimes it just requires tweaking dates by a few days. Indeed, Airfarewatchdog.com lists airfares based on the lowest possible price, assuming that you’re date-flexible, which is why people with specific travel dates often don't find the fares we list.
It's disappointing that Travelocity, Orbitz, Hotwire and Expedia have dumbed-down their flexible date search tools, in part because these searches place a lot of stress on their computer systems (searching for a needle in a haystack is more data-intensive than searching just a specific date). At best, these sites now just offer a one-to-three-day flexible date search.
Here are some tools we like for finding the cheapest flights.
Google has two robust products, with data powered by ITA Software, a fare provider Google now owns.
Google.com/flights is for people who have specific travel dates.
Google.com/flights/explore is for people with flexible dates and flexible itineraries.
It doesn't show fares on Southwest, Allegiant, and a few other airlines, such as Turkish Airlines, but it has most carriers.
Kayak, a meta-search travel site rather than an OTA (the difference between meta search and OTA is explained here), has a robust flexible date search, also powered by ITA Software (see below). First you have to sign up with your email. From the flights tab click on the "more search options" link under "find flights" and then choose Flex month. Specify a departure date and a trip length, either a single number of nights or a range of up to any seven days (such as 6-8 or 18-25). You won’t find Southwest, Allegiant, or Ryanair, but you will find a wide range of destinations, domestic and international. Once in a while, however, you’ll click through on fare found via flexible search and find that it’s not available. That’s just the nature of the beast. And you may not find the best possible routings, since airlines apparently don’t share their entire inventories with third-party sites.
No discussion of flexible date airfare search can exclude ITA Software’s Matrix Airfare Search function, but read why it shouldn’t be relied on as the Holy Grail. We actually like the Google Flights implementation of this data better.
This newish, under-the-radar site is a very cool airfare search product for many reasons. You enter a "to" and "from" airport and then a trip length such as "about two weeks" and you'll see a bar graph showing the fares for dates in that range.
Some airlines have good flexible date search tools as well.
A great page to search is their Best Fare Finder. You have to find your outbound flight date first and then search for the return if flying round-trip.
One of the positive changes United made when it combined the Continental website with its own was to introduce an excellent flexible date function. From the home page “Flight” tab choose a calendar start date, a length of trip, click on “My dates are flexible” choose a length of stay and search. Use the blue forward arrow to the right of the calendar search through United’s entire schedule over 330 days, month by month. Easy.
Delta until fairly recently only allowed +/- 1-3 day search however now it provides a five-week view (you still have to enter specific dates, but the site will propose alternate dates if you use the “calendar view”). American used to have a 30-day flexible date search but got rid of it. However…
Southwest doesn’t list its fares on meta-search or OTA sites, but it does have an excellent flexible date search feature. As the case with other sites mentioned here, Southwest.com doesn’t scream this fact from its homepage. To find the “Shortcut” Low Fare Calendar takes some work, unless you know where it is. It's right here.
Choose a departure and arrival city and a departure and arrival month and you’re all set.
And if you have specific dates, Southwest now has a flexible destination fare map feature. Enter your departure airport and travel dates and you'll see a map of destinations and prices for those dates.
On this super-discounter simply click on the "view 30-day Calendar" buttons to see month-long views of fares.
On Frontier Airlines' website you enter specific dates but the site defaults to a 30-day calendar, which is super helpful.
If you're traveling within Europe, easyJet.com has a simple and effective flexible date tool. Just click on the “Flexible on dates” box to use it. Ryanair.com has one too, similar to the JetBlue model. Interestingly, the “Flexible dates” button is pre-checked, suggesting that Ryanair prefers that you be flexible to find its lowest fares. Their fare finder page is also great and has a map feature similar to Southwest's. AerLingus.com also defaults to flexible dates, searching over a two-week period for both departure and return. Britishairways.com automatically shows a seven-day flexible date range and provides previous week and next day arrows to further search.