Following the Admin Guide, first, you should create a working calendar  (Administration >SLA Calendars > Calendar Management), by clicking on  'Add a calendar'

Create a name

Only the name of the working calendar is a mandatory field

Set the working days/times

• For this calendar, the working days are from Monday to Friday.
• The working period is from 09 am to 05 pm.
• Except for Friday,  where the working period is from 10 am to 05 pm.

Adding the holidays to the calendar

The repeated checkbox allows us to configure the annual holiday ( ex: New year, Christmas...)

You can see the configuration of the new calendar (Simple Working Day)

You can modify or delete the calendar (1)