The fare buckets are re-assessed sometime between around 2am and 5am Central Time (between 4 and 5am is common in my experience). The buckets will change during the day based on sales and cancellations, but if you are looking for more seats to be opened in discount buckets (not based on cancellations during the day), it will most likely happen during that overnight window if at all.

There can also be logic on some routes that makes discount buckets more available at various days in advance if the flight is not projected to be sold out (e.g. at 13-14 days out, 5-6-7 days out), during that overnight window.

Within a day of travel there are more frequent adjustments to fare buckets.