To change the normal work hours in a week, click on the third button from the left on the toolbar. You should see a calendar with 2 green bubbles. Those bubbles designate when the working hours are. If you simply click and drag the bubbles around, you can make the working hours longer, shorter, or go into the weekends as well.
If you want to set certain off-days like Christmas for example, you can click on the drop down menu on the lower left side of the window and select "Work Schedule for Project". This will allow you to select a specific date on the calendar and set the workable hours for that day.