Workdays in Cerebro are determined by work schedules.
Main menu - Tools - Working schedules...
After the Universe is created, all users are set to work 24/7 by default.
"Work schedules" window - "Generic Universe_name user" - default work schedule of users if a user doesn't have a custom schedule set. After you set a schedule, it will be displayed on the Gantt chart and in the Calendar.
It will also be taken into account when calculating task duration.
Read more about work schedules here.