Full answer
Google Flights and airline pricing studies show the "prime booking window" for domestic US flights is 1–3 months out, and for international flights it's 2–6 months out. Booking too early (6+ months) or too late (within 2 weeks) both tend to cost more.
Day of week matters for both booking and flying. Historically, Tuesday and Wednesday departures are cheaper because business travelers drive up Friday and Sunday prices. Similarly, searching for flights on Tuesday or Wednesday afternoons can surface lower fares.
Price alert tools are more reliable than trying to time searches manually. Google Flights, Hopper, and Kayak all offer free alerts — set one for your route and let algorithms track the fare over several weeks before you commit.
Flexibility is the biggest single variable. Being willing to fly one day earlier or later, use a nearby airport, or accept a longer layover can cut fares by 20–40% compared to the most convenient option.