Using Shift Exceptions

Trying to understand an aspect of Shift Exceptions.

Lets say we have an employee who works a Shift 9 to 5 Monday to Friday. I understand that I can create a Shift Exception to indicate that employee is not working on a specific day that they normally work on.

However, what if the employee is going to come in to work on a Saturday, for example. It seems I can't make a Shift Exception that says the employee is available on a day that they do not normally work..