I have seen reference to timing of fare changes in the united system on various threads. Checking daily I do see fare changes of hundreds of dollars from one day to the next and back again. Is there a specific time of day when the fares change (I assume its 5 am ET based on my tracking)? I have tried searching the forum and unfortunately, I have no idea of the key words to use. Any guidance is appreciated. Thank you and happy travels.

Have never heard of an universal time for fare changes. Believe it is an around the clock activity. You may see a specific flight change occur at a specific time or not -- but no general time.
Some discussion that when flight holds (midnight eastern) are released that some lower fare buckets can re-open up.
Also some discussion weekends tend to be more expensive than mid-week but I have not really seen that -- that may be a fare bucket availablity issue.

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).

I don't think that's true. As far as I know airlines do have regularly scheduled change times, but they can also change fares any time they need/want to.