Hi Devashish,
You can do this simply setting StartDate and EndDate property. Like
calExtender.EndDate = DateTime.Today;
calExtender.StartDate = DateTime.Today.AddDays(-6);
here calExtender is Id of the CalendarExtender.
Note: If you are not able to access this property try downloading latest AjaxControlToolkit's dll.
Thanks and Regards,
Rk_Hirpara