In which version do you want to achieve this requirement.

The reason why I asked for PRPC was, I think this is achievable in 7.3.1+ versions because we have this checkbox for disabling weekends in DateTime Control. 
So, I created a calendar in which only Saturday was a business day. After that, when I clicked on "Disable Weekends", I was able to achieve your requirement.

Please refer to attach screenshots for more information.

Hope it helps.

Can you try pasting the below code in userworkform and let me know if this helps.

<style>
tbody[id="controlCalBody"] tr td:nth-child(1), tbody[id="controlCalBody"] tr td:nth-child(2), tbody[id="controlCalBody"] tr td:nth-child(3),tbody[id="controlCalBody"] tr td:nth-child(4),tbody[id="controlCalBody"] tr td:nth-child(5),tbody[id="controlCalBody"] tr td:nth-child(7){
pointer-events:none;
    background: grey;
}
</style>

Regards,

Rachit