They generally will not "change" fares during the course of a given day. What may happen during the course of a day is that lower-priced fares may disappear if all of the seats allocated in that fare class are purchased by someone else. I guess it's possible that lower fares could reappear after having been previously sold out if someone cancels during the day, but I would expect that to be fairly rare.
Sales and discounts frequently are released on Tuesdays, if I recall correctly. And they all would come out basically at once.
I have had fares go down during the day. I have a fair amount of time to check frequently, and in the last few weeks, I had fares go down for my SIL, and today ours went down a little. I was able to snag all of those.
I think it's fares that show up when people rebook. However, today's didn't appear that way, as I checked the fares on the monthly calendars, and they were a little lower.
I too have experienced fare changes during the course of a single day. It was more frequent a year+ ago. I've seen them drop in the morning or in the afternoon. There have been times it was very temporary for only for a few hours and other times the fare remains the same for a day or two. It's usually been a Tues - Thurs situation. Recently, I had a fare drop Sunday night for a few hours - I have not seen that happen in a number of years.
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