The following code sample demonstrates how to change a date's appearance dynamically as the selection changes, based on whether that date is a working day: 

using Infragistics.Win;
using Infragistics.Win.UltraWinSchedule;

Infragistics.Win.Appearance workingDaySelectedAppearance = new Infragistics.Win.Appearance();
Infragistics.Win.Appearance nonWorkingDaySelectedAppearance = new Infragistics.Win.Appearance();

//  Set the properties on the working day and non-working day appearances
this.workingDaySelectedAppearance.BackColor = Color.Green;
this.nonWorkingDaySelectedAppearance.BackColor = Color.LightBlue;

//  Set the UltraCalendarInfo's Tag property to reference the
//  UltraCalendarLook, so we can get that reference easily
//  in the BeforeSelectedDateRangeChange event
this.ultraMonthViewMulti1.CalendarInfo.Tag = this.ultraMonthViewMulti1.CalendarLook;

//  Handle the BeforeSelectedDateRangeChange event
this.ultraMonthViewMulti1.CalendarInfo.BeforeSelectedDateRangeChange += new BeforeSelectedDateRangeChangeEventHandler(CalendarInfo_BeforeSelectedDateRangeChange);

void CalendarInfo_BeforeSelectedDateRangeChange(object sender, BeforeSelectedDateRangeChangeEventArgs e)
{
    //  Get the UltraCalendarInfo and UltraCalendarLook
    UltraCalendarInfo calendarInfo = sender as UltraCalendarInfo;
    UltraCalendarLook calendarLook = calendarInfo.Tag as UltraCalendarLook;

    //  First clear the ones that were set on the last selection
    this.ApplySelectedAppearance( calendarInfo, calendarLook, calendarInfo.SelectedDateRanges, null, null );

    //  Now set the appearance for the new selection
    this.ApplySelectedAppearance( calendarInfo, calendarLook, e.NewSelectedDateRanges, this.workingDaySelectedAppearance, this.nonWorkingDaySelectedAppearance );
}

private void ApplySelectedAppearance(
    UltraCalendarInfo calendarInfo,
    UltraCalendarLook calendarLook,
    SelectedDateRanges selectedDateRanges,
    Infragistics.Win.Appearance workingDaySelectedAppearance,
    Infragistics.Win.Appearance nonWorkingDaySelectedAppearance )
{
    //  Create a dictionary that holds the selected dates and their
    //  respective DayLook objects
    Dictionary<DateTime, DayLook> dayLookList = new Dictionary<DateTime, DayLook>(selectedDateRanges.SelectedDaysCount);

    //  Iterate the date ranges
    foreach( DateRange range in selectedDateRanges )
    {
        //  Iterate each date in the current range, and add the DayLook
        //  for that date to the dictionary
        DateTime date = range.StartDate;
        while ( date <= range.EndDate )
        {
            dayLookList.Add( date, calendarLook.GetDayLook( date, true ) );
            date = date.AddDays(1);
        }

        //  Iterate the dictionary entries and apply the appropriate appearance
        foreach( KeyValuePair<DateTime, DayLook> item in dayLookList )
        {
            bool isWorkDay = calendarInfo.DaysOfWeek[item.Key.DayOfWeek].IsWorkDay;

            Infragistics.Win.Appearance appearance = isWorkDay ?
                workingDaySelectedAppearance :
                nonWorkingDaySelectedAppearance;

            item.Value.Appearance = appearance;
        }
    }

}