Originally Posted by Mr MCO
Typically fares are set after 2:00 am and vary from day to day.... It is done on a complex equation, so if you try one evening and cannot find a low fare, it may be available in the morning.
Can you explain the 2:00 A.M. part?
I work with Orbitz/CheapTickets and our fares change at Midnight in the timezone in which the company is located. Orbitz uses Central (GMT-6) and CheapTickets uses Hawaiian (GMT-10). By "change" I mean that fares which expire on a certain date do so at Midnight.
Fares often have advance purchase requirements, meaning some fares must be purchased 14 days before travel, some 3 days before, and so on. Generally, the closer you get to travel, the more expensive the fares become. It's doubly expensive, because not only do you have more expensive fares, but often the cheaper seats have been sold.
Of course, airlines can and do offer discounted fares on certain dates, times of day, days of the week, etc.