Hello Mehmet,
We are using the standard date and time format strings described
in this article. Note that there is no format string for 2 digit days. Anyway, I can suggest you a different approach to achieve the requirement - you could set the TimeRulerItemTemplateSelector property of the ScheduleView and customize the needed text in the Group TimeRulerItem. You should use HorizontalTimelineGroupTemplate property - I would suggest to create a ValueConverter to convert the day name:
I've attached a simple example to demonstrate the approach. Hope this helps.
Greetings,
Yana
the Telerik team