Technology For the lowest airfare, book a month ahead and on a Sunday By Jamie Biesiada / December 12, 2017 Share 1 Photo Credit: Shutterstock -- Want a cheaper airline ticket? Book at least a month in advance and on a Sunday, a strategy many seem to be employing -- at least in part -- based on an increasing advance-purchase window. Lower average ticket prices in economy class can most often be found by booking more than 30 days before travel, according to the ARC 2018 Air Travel Outlook Report, a collaboration between ARC and Expedia, Inc. For the best prices on international flights, it is best to book on a Sunday and depart on a Thursday or Friday. The best day to travel for domestic economy varied, but Sunday was generally the best day to book the cheapest fare."Despite popular belief in the benefits of last-minute booking, in most parts of the world, no matter where you are headed, economy fares tend to increase as departure date gets closer," the report said. "The most cost-effective booking lead time for an economy flight is one month, or even earlier, from the departure date."The same largely holds true for first class and business class.It appears that corporate travelers are even shifting to longer booking windows, based on data from Egencia customers. Egencia is Expedia's corporate travel agency. "This indicates customers, or their travel managers, are savvy to this book-as-early-as-possible, price-saving trend," the report said.Expedia and ARC, collaborating for the fourth time on this year's report, analyzed ARC flight data from Sept. 1, 2016, to Aug. 31, 2017.