Why do flight prices change so much?
Airlines use revenue management, meaning the same seat is sold at many different prices depending on when it is bought and how demand is developing. Every flight is divided into fare buckets, each with a limited number of seats at a given price. As the cheaper buckets sell out, the displayed price rises.
That mechanism explains most of what looks like erratic behaviour. A price can rise because a handful of seats sold, fall because a bucket was reopened when demand was weaker than forecast, and differ between two searches minutes apart because inventory changed.
Several factors feed the forecast. Historical demand for that route and date, current booking pace against expectation, competitor pricing, events and school holidays, and the mix of business and leisure travellers. Routes dominated by business travel price late bookings very high because those travellers are less price sensitive and book closer in.
The persistent belief that prices are raised because of your search history, via cookies, is not well supported. Airlines price by inventory and demand, not by individual browsing. Prices change between searches because inventory genuinely moves, and searching in a private window is harmless but rarely the explanation.
The practical guidance that survives scrutiny is unexciting. There is no single best day to book. Booking extremely early rarely gets the lowest fare, and booking very late usually costs more. A window of a few weeks to a few months out is typical for leisure routes, with longer for peak periods.
Price alerts on a specific route are more useful than general timing advice, because they respond to the actual inventory rather than to averages.