I am trying to bind a 2D array of buttons arranged in stackpanels to a 2D ObservableCollection...

Yet, I'm afraid I don't understand something very elementary about binding.

My XAML:

    <Window.Resources>
    <DataTemplate x:Key="ItemsAsButtons">
        <Button Content="{Binding}" Height="100" Width="100"/>
    </DataTemplate>
    <DataTemplate x:Key="PanelOfPanels">
        <ItemsControl ItemsSource="{Binding Path=DayNumbers}" ItemTemplate="   {DynamicResource ItemsAsButtons}">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <StackPanel Orientation="Horizontal"/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
        </ItemsControl>
    </DataTemplate>
    </Window.Resources>

...
        <ItemsControl x:Name="DaysPanel" Grid.ColumnSpan="7" Grid.Row="2"
                      ItemTemplate="{DynamicResource PanelOfPanels}"/>

My C# code: The backend:

/// <summary>
/// Window BE for Calendar.xaml
/// </summary>
public partial class Calendar : Window
{

    private CalendarViewModel _vm;
    public Calendar()
    {
        InitializeComponent();
        _vm = new CalendarViewModel();
        this.DataContext = _vm; 
    }
}

The ViewModel:

class CalendarViewModel
{
    CalendarMonth _displayedMonth;
    EventCalendar _calendar;

    public CalendarViewModel()
    {
        _displayedMonth = new CalendarMonth();
    }

    public ObservableCollection<ObservableCollection<int>> DayNumbers
    {
        get
        {
            return _displayedMonth.DayNumbers;
        }
    }
}

I'm trying to populate the buttons with values from CalendarViewModel.DayNumbers - yet the buttons do not appear. I'm clearly doing something wrong with my binding.