Thank you for contacting Telerik Support.
I have performed some local tests, regarding the described inability to apply the custom CssClass on the time slots for the weekends, but it seems to work properly at my end.
Could you inspect the element for the weekend timeslots with your developer tool the preferable browser, just to verify if the CssClass is applied to that slot. If it is applied, but the styles are not - you may want to enforce your selectors, using !important for example.
Hope this would help.
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal
and vote to affect the priority of the items