After returning from the canceled 2020 season, the minor leagues adjusted the schedule to have team’s play six-game series from Tuesday through Sunday, with a near universal day off for travel on Monday. It’s a system that is still being used to this day, now entering year three of doing so.
One of the ways the Pirates tried and eased pitchers back into a full season worth of baseball was to adapt to the schedule and use a six-man rotation. After missing an entire year of competition, this eased pitchers back in by having them pitch once a week.