Off day settings is where you would put all off days that happen during the week. For example spring break, holiday break and so on. 

Go to system setting 
After hitting system settings this screen will come up. 

Go to the other options tab and click on Off Day Settings 

Pick each date you have off of school during the week and hit the green plus sign this will have to be updated each year. 

Once the dates are in you can update all existing incidents by clicking on the update existing incidents.