Currently you can only change the workday status for a day of the week, i.e., not individual days. NA 2010.2 will contain support for designation of non-working days/hours on a per-day basis.

You can make a day of the week a non-working day by setting the IsWorkDay property to false for that day of the week. You can access the day of week objects from the UltraCalendarInfo.DaysOfWeek collection.