Classic

1. Click Your Name | Setup | Company Profile | Business Hours
2. Click New Business Hours and create the business hours setting 
3. Remember to Click Active to allow users to associate the business hours with cases and escalation rules. [Click Active again to deactivate the business hours. You cannot delete business hours.] 
4. Optionally, click Use these business hours as the default to set the business hours as the default business hours on all new cases.

Lightning

1. Click the gear button on the upper right of the page.
2. Select “Setup”.
3. On the Quick Find box, search for Business Hours.
4. Select “Business Hours” (Under Company Settings).
5. Click “New Business Hours” Button.
6. Set the business hours name (Step 1)
7. Click Active. (You can click “Use these business hours as the default” to set this as default)
8. Set the Timezone. (Step 2)
9. Uncheck “24 hours” checkbox to set Business Hours. (Step 3)
10. Click Save.