If you must book late, try to do so at least 21 days out to avoid the steepest fare jumps. When to Fly for the Best Deal The day you fly matters more than the day you book.
Tuesday and Wednesday are consistently the least expensive days for international travel. Book 4 to 10 months ahead
There is no "magic day" to purchase a ticket; modern airline algorithms change prices constantly based on demand, not the day of the week. When Is the Best Time to Buy Plane Tickets? - CLEAR