Days off are designated for the entire company and are currently downloaded to the workload module and are also indicated in the IC Project calendar. To designate days off, go to Settings -> Calendar/ Absences -> Days off. Here, simply click on the desired day to indicate that it is a day off for your company. Deselecting a day to make it a working day again works similarly. Simply click on the day again. 

If you want to speed up the whole process, you can use the Default Days option, which is located in the top left corner. This is a solution that will tick the selected days automatically. This is optimal for deselecting, for example, weekends throughout the year.