Here you can edit your "working" or business hours for use when assigning Resources to Tasks and Service Orders. These are also used when working with Gantt View of Project.
You can set your "From" and "To" hours as well as mark which days you celebrate the weekend.
The Business Holidays tab allows you to set the holiday's your company observes and when creating you can choose whether or not the holiday repeats each year.
You can use the Change Year button to set up holidays that vary on date across multiple years.