Below is the breakdown from Hopper:

  • For domestic and international trips, the cheapest day to depart is Wednesday and the worsts day is Sunday. You can save about $40 on domestic trips and $60 on international trips by leaving on the cheapest day as opposed to the most expensive day.
  • The best day to return from a domestic trip is Tuesdays with savings of about $45.
  • The best day to return from an international trip is Wednesday with savings of about $60.
  • Friday is the most expensive return day for both domestic and international trips.
  • Thursday is the cheapest day to buy domestic and international tickets.  Saturday is the worst day to buy domestic tickets and Sunday is the worst day to buy International tickets. However, savings are only about $10 between the best and worst shopping days for typical domestic routes, and $25 for international markets.

More here.