When it's time to book your flights, being organized pays off. While you could pick up a last-minute deal on a KBP to Ko Samui Airport (USM) flight, prices usually don't get cheaper as the departure date nears. Based on flight pricing and demand studies in 2022, we have found that the sweet spot to book a domestic flight is between 28 – 35 days ahead. For international travel it's between three and four months.