When's the best time to book a flight? With all the recent horror stories, you may facetiously say "never!" Summer is upon us, and millions of people are planning their big vacation of the year. Many, myself included, will involve air travel. So, when's the best time to book your flight. Airfares fluctuate, and booking early is widely considered to be a good idea. The Huffington Post reports, It makes a difference where you're going. Cheapair.com recommends booking domestic flights 54 days in advance. They recommend flights Europe be booked 99 days in advance, the Caribbean, 76 days, and Mexico, 61 days. I started pricing flights for an August trip in March, didn't like what I saw.