<UserControl x:Class="RadControlsSilverlightApp1.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480" xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation">
  <Grid x:Name="LayoutRoot">
        <telerik:RadCalendar  HorizontalAlignment="Left" Margin="136,133,0,0" Name="radCalendar1" VerticalAlignment="Top" />
    </Grid>
</UserControl>

Code Behind

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

namespace RadControlsSilverlightApp1
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
            radCalendar1.SelectionChanged+=new Telerik.Windows.Controls.SelectionChangedEventHandler(radCalendar1_SelectionChanged);
        }

        private void radCalendar1_SelectionChanged(object sender, Telerik.Windows.Controls.SelectionChangedEventArgs e)
        {
            MessageBox.Show("test");
        }
    }
}