public partial class KSDatePicker : UserControl
    {
        #region Properties
        public static readonly DependencyProperty DateProperty = DependencyProperty.Register("SelectedDate", typeof(DateTime), typeof(KSDatePicker), new FrameworkPropertyMetadata(DateTime.Now, OnSelectedDateChanged));

        public DateTime SelectedDate
        {
            get { return (DateTime)GetValue(DateProperty); }
            set { SetValue(DateProperty, value); }
        }

        #endregion
        public KSDatePicker()
        {
            InitializeComponent();
            //Props

            //Events
            DateChangedEvent = EventManager.RegisterRoutedEvent("SelectedDateChanged", RoutingStrategy.Bubble, typeof(RoutedPropertyChangedEventHandler<DateTime>), typeof(KSDatePicker));
            (this.Content as FrameworkElement).DataContext = this;
        }

        #region CallBacks
        private static void OnSelectedDateChanged(DependencyObject _d, DependencyPropertyChangedEventArgs _e)
        {
            DateTime ndt = (DateTime)_e.NewValue;
            KSDatePicker picker = (KSDatePicker)_d;
            picker.SelectedDate = ndt;
        }

        private void ButtonMonthView_Click(object sender, RoutedEventArgs e)
        {
            if (popup1.IsOpen)
                popup1.IsOpen = false;
            else
                popup1.IsOpen = true;
        }
        #endregion

        public static RoutedEvent DateChangedEvent;
        public event RoutedPropertyChangedEventHandler<DateTime> SelectedDateChanged
        {
            add { AddHandler(DateChangedEvent, value); }
            remove { RemoveHandler(DateChangedEvent, value); }
        }

        private void ButtonNextDay_Click(object sender, RoutedEventArgs e)
        {
            OnSelectedDateChanged(this, new DependencyPropertyChangedEventArgs(DateProperty, SelectedDate, SelectedDate.AddDays(1)));
        }

        private void ButtonPreviousDay_Click(object sender, RoutedEventArgs e)
        { OnSelectedDateChanged(this, new DependencyPropertyChangedEventArgs(DateProperty, SelectedDate, SelectedDate.AddDays(-1))); }
    }

<UserControl x:Class="KsCustomControls.KSDatePicker"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             mc:Ignorable="d" 
             d:DesignHeight="100" d:DesignWidth="350" 
             MaxHeight="100" MaxWidth="350"
             Name="Picker">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"></ColumnDefinition>
            <ColumnDefinition Width="3*"></ColumnDefinition>
            <ColumnDefinition Width="*"></ColumnDefinition>
        </Grid.ColumnDefinitions>
        <Button Grid.Column="0" Name="ButtonPreviousDay" Content="{TemplateBinding Calendar.SelectedDate}" Click="ButtonPreviousDay_Click"></Button>
        <Border Grid.Column="1">
            <Viewbox HorizontalAlignment="Stretch" VerticalAlignment="Stretch" Name="vb">
                <TextBox Name="ButtonMonthView" PreviewMouseLeftButtonUp="ButtonMonthView_Click" Text="{Binding Path=SelectedDate, StringFormat=dd.MM.yy, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}" IsReadOnly="True" HorizontalContentAlignment="Stretch" VerticalContentAlignment="Stretch"/>
            </Viewbox>
        </Border>
        <Button Grid.Column="2" Name="ButtonNextDay" Margin="0" Click="ButtonNextDay_Click" ></Button>

        <Popup Name="popup1"  VerticalOffset="20" StaysOpen="False" PopupAnimation="Scroll"  AllowsTransparency="True" Placement="Bottom" PlacementTarget="{Binding ElementName=ButtonMonthView}" IsOpen="False">
            <Grid>
                <Calendar Name="Calendar1" SelectedDate="{Binding Path=SelectedDate, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}"></Calendar>
            </Grid>
        </Popup>
    </Grid>
</UserControl>

<Window x:Class="Tester.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ksControls="clr-namespace:KsCustomControls;assembly=KsCustomControls"
        Title="MainWindow" Height="350" Width="525" Name="mw">
    <Grid>
        <TabControl HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
            <TabItem Header="DatePicker">
                <Grid>
                    <!--<ksControls:TEST></ksControls:TEST>-->
                    <ksControls:KSDatePicker SelectedDate="{Binding date, Mode=TwoWay}" SelectedDateChanged="KSDatePicker_SelectedDateChanged"></ksControls:KSDatePicker>
                    <Label Content="{Binding date}"  HorizontalAlignment="Left" Margin="49,24,0,0" VerticalAlignment="Top" Height="30" Width="136"/>
                </Grid>
            </TabItem>
            <TabItem Header="TabItem">
                <Grid Background="#FFE5E5E5"/>
            </TabItem>
        </TabControl>

    </Grid>
</Window>

    public partial class MainWindow : Window
    {
        public DateTime date { get; set; }
        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
            date = DateTime.Now;
        }

        private void KSDatePicker_SelectedDateChanged(object sender, RoutedPropertyChangedEventArgs<DateTime> e)
        {
            MessageBox.Show(date.ToString());
        }
    }

#region
using System;
using System.Windows;
using System.Windows.Controls;



#endregion


namespace KsCustomControls
{

    public partial class KSDatePicker : UserControl
    {
        public KSDatePicker()
        {
            InitializeComponent();
            (Content as FrameworkElement).DataContext = this;
        }

        #region Properties

        #region SelectedDate
        public static readonly DependencyProperty DateProperty = DependencyProperty.Register("SelectedDate", typeof (DateTime), typeof (KSDatePicker), new FrameworkPropertyMetadata(DateTime.Now, OnSelectedDateChangedCallback));

        // RE: changed event identifier & registration
        public static readonly RoutedEvent SelectedDateChangedEvent =
            EventManager.RegisterRoutedEvent(
                "SelectedDateChanged",
                RoutingStrategy.Bubble,
                typeof (RoutedPropertyChangedEventHandler<DateTime>),
                typeof (KSDatePicker));

        public DateTime SelectedDate { get { return (DateTime) GetValue(DateProperty); } set { SetValue(DateProperty, value); } }

        // RE: changed event
        public event RoutedPropertyChangedEventHandler<DateTime> SelectedDateChanged { add { AddHandler(SelectedDateChangedEvent, value); } remove { RemoveHandler(SelectedDateChangedEvent, value); } }

        // RE: use a protected virtual to raise event
        protected virtual void OnSelectedDateChanged(DateTime oldValue, DateTime newValue)
        {
            var e =
                new RoutedPropertyChangedEventArgs<DateTime>(oldValue, newValue);
            e.RoutedEvent = SelectedDateChangedEvent;
            RaiseEvent(e);
        }

        private static void OnSelectedDateChangedCallback(DependencyObject _d, DependencyPropertyChangedEventArgs _e)
        {
            var ndt = (DateTime) _e.NewValue;
            var picker = (KSDatePicker) _d;
            picker.SelectedDate = ndt;
            picker.OnSelectedDateChanged((DateTime) _e.OldValue, (DateTime) _e.NewValue);
        }
        #endregion

        #endregion

        private void ButtonNextDay_Click(object sender, RoutedEventArgs e) { OnSelectedDateChangedCallback(this, new DependencyPropertyChangedEventArgs(DateProperty, SelectedDate, SelectedDate.AddDays(1))); }

        private void ButtonPreviousDay_Click(object sender, RoutedEventArgs e) { OnSelectedDateChangedCallback(this, new DependencyPropertyChangedEventArgs(DateProperty, SelectedDate, SelectedDate.AddDays(-1))); }

        private void ButtonMonthView_Click(object sender, RoutedEventArgs e)
        {
            if (popup1.IsOpen)
                popup1.IsOpen = false;
            else
                popup1.IsOpen = true;
        }
    }

}

<Window x:Class="Tester.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:ksControls="clr-namespace:KsCustomControls;assembly=KsCustomControls"
        xmlns:tester="clr-namespace:Tester"
        Title="MainWindow" Height="350" Width="525" Name="mw">
    <Grid>
        <TabControl HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
            <TabItem Header="DatePicker">
                <Grid Margin="0,0,0,150">
                    <ksControls:KSDatePicker SelectedDate="{Binding date, Mode=TwoWay}" SelectedDateChanged="KSDatePicker_OnSelectedDateChanged"></ksControls:KSDatePicker>
                    <Label Content="{Binding date, Mode=OneWay}"  HorizontalAlignment="Left" Margin="49,24,0,0" VerticalAlignment="Top" Height="30" Width="136"/>
                </Grid>
            </TabItem>
            <TabItem Header="TabItem">
                <Grid Background="#FFE5E5E5"/>
            </TabItem>
        </TabControl>

    </Grid>
</Window>

    public partial class MainWindow : Window
    {
        public DateTime date { get; set; }
        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
            date = DateTime.Now;
        }
        public event PropertyChangedEventHandler PropertyChanged;

        public void RaisePropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        private void KSDatePicker_OnSelectedDateChanged(object _sender, RoutedPropertyChangedEventArgs<DateTime> _e)
        {
            //MessageBox.Show("HURAAAAA!");
            RaisePropertyChanged("date");
        }
    }