Create a WPF project and create a UserControl. The UserControl contains one calendar.

<UserControl x:Class="WpfApplication6.UserControl1 "
 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" Name="user"
 d:DesignHeight="158" d:DesignWidth="183">
 <Grid Height="154" Width="186">
 <Calendar Height="150" HorizontalAlignment="Left" Name="calendar1" VerticalAlignment="Top" Width="189" />
 </Grid>
</UserControl>

In the main window create a popup xaml code.Create a reference of theUserControl. Then call the UserControl inside the popup.

<Window x:Class="WpfApplication6.MainWindow"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:usr="clr-namespace:WpfApplication6"
 Title="MainWindow" Height="350" Width="525">
 <StackPanel>
 <ToggleButton Name = "button" Height="45" Width="154" Foreground="Green">
 <ToggleButton.Template>
 <ControlTemplate TargetType="ToggleButton">
 <TextBlock> Click here to view the popup</TextBlock>
 </ControlTemplate>
 </ToggleButton.Template>
 </ToggleButton>
 <Popup IsOpen="{Binding IsChecked, ElementName=button}">
 <Border Background="Green">
 <usr:UserControl1></usr:UserControl1>
 </Border>
 </Popup>
 </StackPanel>
</Window>