The following code sample demonstrates how to solve the problem you describe here using the IUIElementDrawFilter interface:

using nsMonthViewMulti = Infragistics.Win.UltraWinSchedule.MonthViewMulti;

XDayDrawFilter drawFilter = new XDayDrawFilter( this.ultraMonthViewMulti1 );
drawFilter.AddDate( DateTime.Today.AddDays(7) );
this.ultraMonthViewMulti1.DrawFilter = drawFilter;

#region XDayDrawFilter
public class XDayDrawFilter : IUIElementDrawFilter
{
    private Dictionary<DateTime, object> dates = null;
    private UltraMonthViewMulti control = null;

    public XDayDrawFilter( UltraMonthViewMulti control ) { this.control = control; }

    public void AddDate( DateTime date )
    {
        date = date.Date;

        if ( this.Dates.ContainsKey(date) == false )
        {
            this.Dates.Add( date.Date, null );

            if ( this.control != null )
                this.control.Refresh();
        }
    }

    public void RemoveDate( DateTime date )
    {
        date = date.Date;

        if ( this.Dates.ContainsKey(date) )
        {
            this.Dates.Remove( date.Date );

            if ( this.control != null )
                this.control.Refresh();
        }
    }

    private Dictionary<DateTime, object> Dates
    {
        get
        {
            if ( this.dates == null )
                this.dates = new Dictionary<DateTime,object>(0);

            return this.dates;
        }
    }

    #region IUIElementDrawFilter Members

    bool IUIElementDrawFilter.DrawElement(DrawPhase drawPhase, ref UIElementDrawParams drawParams)
    {
        Graphics gr = drawParams.Graphics;
        Rectangle bounds = drawParams.Element.RectInsideBorders;
        using ( Pen pen = new Pen(Color.Red) )
        {
            gr.DrawLine( pen, new Point(bounds.Left, bounds.Top), new Point(bounds.Right - 1, bounds.Bottom - 1) );
            gr.DrawLine( pen, new Point(bounds.Left, bounds.Bottom - 1), new Point(bounds.Right - 1, bounds.Top) );
        }

        return true;
    }

    DrawPhase IUIElementDrawFilter.GetPhasesToFilter(ref UIElementDrawParams drawParams)
    {
        nsMonthViewMulti.DayUIElement dayElement = drawParams.Element as nsMonthViewMulti.DayUIElement;

        if ( dayElement != null && this.Dates.ContainsKey(dayElement.Date) )
            return DrawPhase.AfterDrawElement;
        else
            return DrawPhase.None;
    }

    #endregion
}
#endregion XDayDrawFilter