The advance-purchase window is a tendency, not a rule

Treat the sections below as a sequence. With booking horizons, getting the early decisions right makes the later ones much easier.

Where the folklore comes from

Every so often a study of aggregated bookings produces a headline number of weeks, and that number circulates long after its data has aged. The averages behind those headlines combine business routes, holiday routes and seasonal ones, and the average of very different curves describes none of them.

A single best day to buy would also be self-defeating, because if everybody believed it the demand on that day would move the price. What survives the averaging is a shape rather than a date, and the shape is genuinely useful once you stop looking for a rule. Treat any specific number of days you read as a summary of other people journeys rather than a prediction about your own.

The shape of the curve

Fares tend to open at a middling level when the schedule loads, drift sideways through a long middle period, then rise as departure nears. The middle stretch is wide, which is why booking a little earlier or later within it usually makes far less difference than people expect. The rise at the end is not a penalty for lateness but the natural result of the cheap buckets having sold to everybody else.

For most people, the occasional late fall happens only when a flight is selling behind its forecast, which on a popular weekend route is genuinely uncommon. So the practical rule is to buy anywhere in the flat middle once your dates are certain, and to stop optimising after that.

Why the curve differs by route

A route full of people travelling for work carries late demand that pays whatever is asked, so the end of its curve rises very steeply indeed. A route serving a leisure destination fills earlier and flatter, because the people flying it plan around fixed weekends and school terms. A route with several competing carriers behaves differently again, since a rival empty seat puts a ceiling on what anybody can charge.

On an ordinary week, you cannot see any of this directly, but you can infer it from who obviously uses the route and how many operators serve it. That inference is more reliable than any universal booking rule, because it is about the actual flight rather than about aggregate statistics.

What certainty is worth

The main cost of booking early is not money but commitment, since a cheap seat you cannot use is worse value than a dear one you can. The main cost of booking late is that you are buying into the steep part of the curve, where each further week tends to cost more. Between those, the sensible moment is as soon as the weekend is genuinely fixed in your life rather than merely likely to happen.

Where it helps most, for a two-night trip that moment often arrives later than for a long holiday, because there is so much less to arrange around it.

Deciding whether the dates are truly fixed is therefore a more productive question than deciding how many weeks ahead to buy.

Seasonal and event distortions

Dates attached to public holidays, school breaks or large local events behave as though the flat middle of the curve had been cut short. On those dates the flight fills from an early stage, so the price starts rising while an ordinary weekend would still be sitting flat. Checking a destination calendar before choosing dates costs a few minutes and explains a price that would otherwise look inexplicable.

On an ordinary week, if the event is the reason you are going, accept the pricing as part of the trip rather than fighting it with timing tricks. If it is not, shifting the weekend to either side of the peak frequently returns you to the ordinary curve altogether.

None of this is a substitute for talking to a clinician if something feels wrong.

A workable approach

Look early to learn what a normal price looks like on your dates, without any intention of buying at that first look. Fix the dates in your life, not merely in your head, by clearing the Friday or the Monday that you are going to need. Buy once the dates are real, take the price that the flat middle offers, and then stop watching the route afterwards.

Accept that you will sometimes see a lower number later, and that the alternative strategy loses considerably more often than it wins. The goal is a trip that happens on the days you wanted, which is worth more than a marginal improvement on the fare.

The takeaway

Buy when the weekend is real, not when a rule of thumb says to.

Pick the one that costs you least, and let the rest wait.