You may have a favorite day to fly ... but you should always buy those airline tickets on Sunday. Expedia analyzed millions of flights and airfare data and found that booking a flight on a Sunday could save you up to 15% on your total airfare.Whatever you do, don't click "buy" on Friday, which is the most expensive day to make your travel arrangements. End of the week booking could mean paying 5-15% more than other days because Friday is notorious for last-minute travel bookings for an upcoming weekend. Business travelers are getting their flights booked for the next week as well.