Several companies work on a five and half day a week schedule. Example: 8am-4pm from Monday to Friday, and 8am-12noon on Saturday. Currently, the shift management functionality do not support the 5.5 work-day use case.
Ideally, when an user assigns the shift to an employee, the shift assignment doctype should display days of the week and along side the shifts that can be assigned on those days. The entire weekly shift schedule should have a start-date and end-date (optional). This will enable shift assignments for many companies who operates on a 5.5 day shift. Here is a shift creation form, which I have designed for reference.
I am not aware whether there are ways to address this in V12/V13? Documentation do not specify this use case.