According to Tom Parsons of BestFares.com,

During the business week [Monday through Friday] airlines generally update their fares three times a day [12:30, 5:00 and 8:00 P.M., E.S.T.]. On weekends they generally update at 5:00 P.M. on Saturdays and 3:00 P.M. on Sundays. When a major airline starts a fare war, it generally attempts to take advantage of the 8:00 P.M. weekday or 3:00 P.M. Sunday "update" so the competition is prevented from matching fares until the next "update" i.e., [12:30 P.M. the following day]. This gives the sale originator a 24-hour price and advertising advantage over the competition.

Hope this helps!