I know AirfareWatchdog has mentioned in the past that airlines can change fares once a day around 5pm. Here's the quote from the site:

Some of the best fares appear on Saturdays

Although they're much less common than in past years, probably because the airines have installed new software to catch them, some of the best fares show up on Saturday mornings. This is probably because airlines can only change fares once a day (around 5 PM) on Saturday and Sunday. So, if you're an airline fare analyst, and you're trying to sneak one over your competition, you'll do it with the last fare change on Friday night, which appears in Travelocity, Orbitz, etc. around 1 or 2 AM on Saturday morning. Your competition can't match your sale fares until the 5 PM Saturday update (those fares appear in Travelocity around 8 or 9 PM that night).