The best deals on summer airline tickets are historically offered approximately 3 1/2 months before a departure date, as airlines attempt to determine their demand for summer travel, reports Rick Seaney at Fare Compare. Travelers looking to purchase airfare at the time will often find even cheaper rates on Tuesdays after 3 p.m., Eastern Time, after most airlines have introduced their sales for the week. As these sales don't last long, travelers should complete their purchase by Thursday, before the sale ends, to ensure the best rates.