How we know: Sources are listed at the end. The mechanics below are standard airline revenue-management practice as described in the cited references; the booking-window figures are from Google Flights' published data, not Wildly's.
The same seat on the same flight can cost one price on Monday and a different one on Wednesday. That is not randomness and it is not your browser cookies. It is the visible surface of a pricing system airlines have refined for four decades. Once you understand its four moving parts, most "mysterious" price changes become predictable enough to work with.
Part 1: a flight is sold in buckets, not at a price
Every flight's cabin is divided into booking classes, each identified by a letter and each with its own price and rules (Airline reservations system). The airline decides how many seats to make available in each class. The cheapest classes get a handful of seats; the most flexible, most expensive classes get many.
When you search, you see the lowest class that still has seats for your dates. When the cheap class sells out, the displayed price jumps to the next class up. That is the single most common reason a fare "went up overnight": nothing was repriced, the cheap bucket simply closed. It also explains why fares sometimes fall: if bookings run behind forecast, the airline reopens a cheaper bucket.
Part 2: revenue management decides how many seats go in each bucket
Airlines run software that continuously compares bookings against a forecast for each departure and adjusts bucket availability accordingly. The goal is to protect enough seats for late, high-paying demand while filling the rest with earlier, price-sensitive travellers. As Wikipedia's summary of the practice puts it, airlines "use specialized software to monitor how seats are reserved and react accordingly," keeping "a specific number of seats in reserve to cater to the probable demand for high-fare seats" (Yield management).
Two consequences for you:
- Fares tend to rise as departure approaches, because the model expects late bookers to be less price-sensitive. Google Flights' data puts the typical low for domestic US fares at 21–52 days before departure and for US–Europe around 94 days out (Thrifty Traveler on Google Flights data).
- Fares fall when a flight is under-booked relative to plan. That is where sales and quiet mid-cycle drops come from, and why they are commoner on off-peak dates and competitive routes.
Part 3: competition and matching
On routes with several carriers, a fare cut by one is usually matched by the others within hours to a day, because the matching is largely automated. That makes competitive trunk routes (New York–Paris, San Francisco–Tokyo, Chicago–London) more volatile than routes with a single operator, and volatility is what makes watching a route worthwhile. See the current fare on JFK to Paris, SFO to Tokyo or Chicago to London.
Part 4: distribution, and why sites disagree
Fares and rules are filed centrally (through ATPCO and SITA) and distributed to global distribution systems and, from there, to agencies and metasearch (Airline reservations system). Airlines also sell some fares only through their own sites or through specific partners. Caching and refresh timing differ by channel, so two sites can show different prices for the same flight for a few hours. When they disagree, the airline's own site is usually the ground truth.
Myths worth dropping
- "The airline raised the price because I searched." Prices are set by bucket availability and the revenue model, not by your search history. What you saw was a bucket closing, sometimes because other shoppers bought while you deliberated.
- "Book on Tuesday." In Google Flights' data the cheapest day of the week to purchase was only about 1.3% cheaper than the dearest; the day you fly matters far more (midweek about 13% cheaper) (Thrifty Traveler on Google Flights data).
- "There is a perfect booking window." There are typical windows by route type, wide ones. Use them as a plan, not a rule.
How to use all this
- Shop the flight, not the day. Compare adjacent days and nearby airports; you are hunting for open cheap buckets.
- Start early enough to see movement. Months out for international travel.
- Decide your price in advance and let an alert watch the buckets. Set a price alert at a realistic target; when a bucket reopens below it, you hear about it the same day.
- When you see a fare you would be happy with inside the typical window, book. The system is designed so that waiting past that point costs more often than it saves.
FAQ
Why did the price change while I was booking? The last seat in that fare bucket sold, or the airline's system refreshed availability. Reloading will not bring it back.
Do incognito windows get cheaper fares? No. They can avoid a cached price on a metasearch site, which is why people occasionally see a different number, but the airline's inventory does not know who you are.
Why is the return sometimes more expensive than the outbound? Each direction is priced from its own buckets and demand; Sunday and holiday-adjacent returns are contested.