When is the cheapest time to book a flight?

The useful version of this answer is a booking window, not a magic hour. Airline fare studies consistently point to roughly one to three months before departure for domestic trips and about two to eight months for international, with the worst prices coming inside two weeks. That window exists because airlines release seats in fare buckets and raise prices as cheap inventory sells and departure nears, when business travelers with expense accounts start buying. The old advice to book at a precise moment on Tuesday afternoon is obsolete, modern airline pricing is algorithmic and continuous, and it does not wait for a weekly sale ritual.

What still moves the price is when you fly rather than when you buy. Midweek departures, Tuesday and Wednesday, are consistently cheaper than Friday and Sunday, early morning and late night flights cost less, and shifting travel by a day or two around holidays saves substantially. Being flexible on airport, some metro areas have two or three, is often worth more than any booking trick. Practical tactics: use a flexible-date search to see a whole month at once, set fare alerts on the routes you care about, book directly with the airline when the price is equal so changes are simpler, and know that US carriers are required to let you cancel most bookings within 24 hours of purchase for a full refund, which lets you lock a fare and keep looking.