<ResourceDictionary>
        <Style TargetType="{x:Type Neptune:Calendar}">
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Neptune:Calendar}">

                    <!--Some header items and other misc controls....-->

                        <DockPanel>

                            <!--Calendar-->
                            <ListBox ItemsSource="{Binding Days}" Background="{x:Null}"
                            SelectedItem="{Binding SelectedDay, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"

                        </DockPanel>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>

public class Calendar : Control
    {
        public ObservableCollection<Day> Days { get; set; }

        //SelectedDay Property
        public static readonly DependencyProperty SelectedDayProperty =
        DependencyProperty.Register("SelectedDay", typeof(Day), typeof(Calendar),
        new PropertyMetadata(null, OnSelectedDayChanged));

        public Day SelectedDay
        {
            get { return (Day)GetValue(SelectedDayProperty); }
            set { SetValue(SelectedDayProperty, value); }
        }

        private static void OnSelectedDayChanged(DependencyObject pager, DependencyPropertyChangedEventArgs e)
        {
            Calendar d = pager as Calendar;
            //MessageBox.Shoy.Date.ToShortw(d.SelectedDaDateString());///THIS SHOWS CORRECT SELECTED DATE!!!!
            d.SetValue(ThisDayProperty, d.SelectedDay);
        }

<Neptune:Calendar Grid.Row="1" x:Name="Calendar" Margin="0,10,0,0"
   ThisDay="{Binding DaySelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged,
    RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}">