(Via Upgrade Travel Better):
I’m frequently asked how far in advance a person should buy a ticket to get the greatest savings. I’ve always believed that booking way, way, way in advance was not the best course of action. Turns out my hunch was correct. Rick Seaney, who runs Farecompare, says that their research has found that booking 3-4 months out generally yields the best deals. His theory is that because Southwest only allows bookings about 4-5 months out, that other competitors won’t match until Southwest has announced their pricing.
International is a different story — pay attention to fare sales to grab the best deals. International fare sales can come more than 6 months out (or less than 2 months out) – international fares are vastly more complex than domestic fares. Still, I rarely suggest booking more than a few months out, even for international travel, if you’re looking for the best deals.