We’ve heard it for years: Search for plane tickets during the middle of the week, on Tuesdays and Wednesdays, to get the best deals. But new data suggests that Sunday is actually the day you’re likely to score the cheapest fares.

Airlines Reporting Corporation, a company that processes $84 billion worth of tickets annually for travel agencies, studied domestic and international ticket sales for a 19-month period between January 2013 and July 2014. According to the study, the lowest average price of $432 was on Sundays, followed closely by Saturday at $439. The most expensive day of the week? Monday, at $503.

weeksummary Summary statistics for U.S. domestic flights by day of week. All advanced purchased days are included. Photo: ARC

The study, which parsed tickets purchased through online and traditional travel agencies but not from airlines directly, also found that the lowest prices occurred 57 days (about eight weeks) before departure for domestic trips and 171 days (about 24 weeks) before departure for international trips.

avg-domestic Average ticket price vs. advanced purchase days for U.S. domestic flights showing average ticket price between 300 Days and departure. Average ticket prices changes very little between 100 and 50 days prior to departure. Photo: ARC

And savings were significant: You save about 19 percent if you buy a domestic round-trip ticket during that sweet spot and 27 percent on international round-trip fares.

avg-intl Average ticket price vs. advanced purchase days for international flights showing average ticket price between 300 days and departure. Average ticket price changes very little between 250 and 150 days prior to departure. Photo: ARC

"This latest study by ARC is significant because it reveals that not only have the lowest airfares shifted from six to eight weeks out for domestic travel, but the savings are markedly greater on a percentage basis," said Chuck Thackston, managing director of enterprise information management at ARC. "It was also interesting to see that the data showed the least expensive tickets were purchased on a Sunday as opposed to Tuesday, which is a common belief.”

But ARC’s research doesn’t mean you shouldn’t look for deals mid-week. Yapta, a firm that tracks declines in airfare prices, found that the biggest price drops occur on Tuesdays (by 21 percent) and Wednesdays (by 19 percent). That’s likely because of fare sales that are typically advertised early in the week, say airline pricing execs.

Finally, there’s one more piece of new data that’s important to note when scouring for the best airfare deal. According to travel search site Hopper, Americans spend about 12 days searching for airfare before they finally book. The problem, though, is that while you’re scouring Kayak and Travelocity and every other site in between, prices rise by an average of 5 percent during that period. What’s more, there’s a one in six chance that the price of your ticket will drop by at least 20 percent within 24 hours of buying it.

But don’t settle for buyer’s remorse. A little-known U.S. Department of Transport policy that requires all domestic and foreign airlines to offer a 24-hour free cancellation policy can come in quite handy. They may not advertise it, but American, Delta, United, US Airways, JetBlue, Southwest, Spirit and Frontier all comply with those guidelines. So cancel, rebook, and enjoy the savings.