DL's algorithms seem a bit out of whack lately due to constant schedule changes. The last month or so the pattern has been very similar: Friday night DL makes schedule changes. Saturday, some flights have abnormally high inventory available and you can snag some really good fares. As they swap pax between flights that have been cx'd, inventory disappears and by Sunday everything is a B or Y fare. Tuesday through Thursday things settle out and "normal" fares appear, but some flights that are either at risk or may need to take pax from a flight that is at risk may only be selling at a B or Y fare even if the flight is empty.