The price of flights usually increases one month prior to the date of departure. Flights are usually cheapest between three and seven weeks prior to departure.
This answer has some exceptions, notably the winter holidays. If the trip is planned during a holiday, the wisest choice is to purchase as far in advance as possible. An analysis of approximately 1.5 billion airfares conducted by CheapAir.com, purchased between one and 320 days in advance of flights, revealed that the best time to book on average is 49 days in advance. In 2014, that number was 47.
The Volatility of Airfare Prices
There is plenty of variability associated with the costs of flights. This variability is ultimately driven by supply and demand. However, booking later than 49 days in advance uniformly carries higher risk of an expensive ticket than booking early.
CheapAir lays out five snippets of advice for booking flights from their analysis:
• Prices are volatile over time. There are plenty of opportunities for great deals in the lead-up to departure. However, prices almost always spike 14 days prior to departure.
• There are always some outliers, but airfare is on average always more expensive when it’s purchased at the last minute.
• The time between a purchase that is too early and a purchase that is too late will almost always have the best value, because prices are higher on average just as flights open and just before they close.
• One to four months before departure is the best booking window.
• Summer and holiday travel are exceptions to the rule.