Except as provided in subsection (b) of this section, an employee may use personal leave after notice to the employee's immediate supervisor. If an employee is scheduled to work on a holiday or is employed in a unit that provides a service continuously on a 7-day-a-week basis, the employee may use personal leave only after obtaining approval from the employee's appointing authority. If the request is made to observe a religious holiday, the appointing authority may deny the request only if: the employee's unit provides a service continuously on a 7-day-a-week basis; the denial is necessary because of a critical shortage of staff in the unit; and no reasonable accommodation to the employee's request can be made. If the request is made for any other reason, the appointing authority may deny the request only if the denial is necessary because of a critical shortage of staff in the employee's unit.