How early should you buy airline tickets to get the lowest price?

To find the best deal on an airline ticket, when you purchase your ticket matters, but how much? The lowest domestic airfares were bought eight weeks beforehand and the lowest international fares were purchased 24 weeks beforehand, according to a report, released Wednesday from a company that analyzed 19 months of airfare purchases.

Airlines Reporting Corp. looked at data for tickets bought between January 2013 through July 2014 to figure out the sweet spots for lowest fares. At the eight- and 24-week points, the report says, fliers saved 19% on domestic flights and 27% on international flights.

The company also found that the least expensive air tickets were purchased on Sundays for just about any destination.

“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,” Chuck Thackston, managing director of enterprise information management, said in the statement.

“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 here’s the big caveat: The data is based on past purchases, and the past doesn’t really predict the future when it comes to airfare shopping.

Still, it can’t hurt to use this data as a guide to checking dates and days of the week when you’re searching for low airfares.

Also the coming Thanksgiving and Christmas holidays add extra angst about prices. Travel editor Catharine Hamm’s “On the Spot” column about ticket-buying trends from Orbitz says:

“The biggest surprise — and we don’t mean that in a happy way — is that if you’re looking for the best day to book the lowest-priced airfare for Thanksgiving or Christmas, you’ve already missed it. It was Wednesday [Oct. 8] for Christmas and Friday [Oct. 10] for Thanksgiving. (Yes, Christmas before Thanksgiving. That may be because more people travel at Christmas, the Orbitz study said.)”

In the 19-month study from ARC, average round-trip domestic ticket prices varied by days of the week. As the report says, Sunday was the cheapest, at $432, and Mondays were most expensive, at $503.