So which day of the week is the best to buy discounted airline tickets? Ten years ago, it used to be early Wednesday morning, actually one minute after midnight. Not anymore.

Google Flights looked at four years of booking data and discovered the most expensive day to book a flight is Sunday, and the least expensive day is now Tuesday. One other discovery, flights cost about 13% less for Monday to Wednesday departures.

For holiday travel, make note for next year. Book 51 days ahead for Christmas travel, and 35 days in advance for Thanksgiving travel.