Finally, thanks to twitter, I have an answer. Here it is:
According to the latest studies, the best time to buy tickets is on Tuesdays at 3:00 pm ET. That’s when airlines tend to release their promotional fares (deals which are sometimes pulled, if not sold out, by the end of the week). How far in advance should you book? Eight weeks in advance of your trip. After that, fares tend to rise steadily as you get closer to your departure date.I don't travel often. But when I do, I like to do it as cheaply as possible. Hopefully this advice will help.