In the Business Hours section of the Setup menu, you will be able to enter in working hours and holidays for your organization and its different sites. These are important when setting up your service desk and creating SLA rules so that they accurately reflect when your technicians are actually in the office. This way, there will be no breaches in SLAs pertaining to "time to assign" or "time to first response" if the technician is not there. The picture below shows where to set these business hours up.
The other section associated to Business Hours is the scheduled time off section which outlines the holidays that your organization observers. You can also import more holidays if need be shown in the picture below.
When building out your SLAs you can choose to apply your business hours. This is how you can make sure that no breaches will occur when no one is in the office. The picture below shows where to apply your business hours:
For more information around creating your business hours, please see the video below: