There are different kind of marketplace modules that can provide you with working days, or you can do it yourself ofcourse, with the functions mendix give. Based on the working days you can assign the working hours per day and then filter on month. 

this is maybe a helpful module

https://marketplace.mendix.com/link/component/110117 

here you can find formula and function to help you

https://docs.mendix.com/refguide/xpath-weekday-from-datetime/#:~:text=The%20weekday%2Dfrom%2DdateTime(),Sunday%2C%207%20%3D%20Saturday).