<Window.Resources>
  <DataTemplate x:Key="foo">
    <Border
      x:Name="MonthBorder"
      Background="Red"
      Margin="3"
      Width="{Binding Width}">
      <TextBlock
        Margin="3"
        Text="{Binding Name}"/>
    </Border>
  </DataTemplate>
</Window.Resources>
<ListBox
  x:Name="lbMonths"
  ItemsSource="{Binding Months}"
  ItemTemplate="{StaticResource foo}"
  VirtualizingStackPanel.IsVirtualizing="False"/> <!-- отключает виртуализацию -->

public partial class MainWindow
{
  public MainWindow()
  {
    InitializeComponent();
    var sd = new DateTime(2013, 1, 1);
    Months = new List<Month>();
    Months.AddRange(Enumerable.Range(0, 12).Select(n =>new Month
    {
      Name = sd.AddMonths(n).ToString("MMMM", CultureInfo.InvariantCulture)
    }));
    DataContext = this;
    lbMonths.Loaded += AdjustItemsSize;
  }

  public List<Month> Months { get; private set; }

  void AdjustItemsSize(object sender, RoutedEventArgs e)
  {
    var maxLength = Months.Max(m =>
    {
      var itemContainer = lbMonths.ItemContainerGenerator.ContainerFromItem(m);
      if (itemContainer==null) return 0;
      var cp = itemContainer.GetVisualChild<ContentPresenter>();
      if (cp==null) return 0;
      var border = cp.ContentTemplate.FindName("MonthBorder", cp) as Border;
      return border == null ? 0 : border.ActualWidth;
    });
    if (maxLength==0) return;
    Months.ForEach(m=>m.SetWidth(maxLength));
  }
}

public class Month: INotifyPropertyChanged
{
  public Month()
  {
    Width = Double.NaN;
  }

  public void SetWidth(double value)
  {
    Width = value;
    if (PropertyChanged!=null)
      PropertyChanged(this, new PropertyChangedEventArgs("Width"));
  }

  public string Name { get; set; }
  public double Width { get; set; }
  public event PropertyChangedEventHandler PropertyChanged;
}

public static class DependencyObjectExtensions
{
  public static T GetVisualChild<T>(this DependencyObject parent) where T : Visual
  {
    var child = default(T);
    var numVisuals = VisualTreeHelper.GetChildrenCount(parent);
    for (var i = 0; i < numVisuals; i++)
    {
      var v = (Visual)VisualTreeHelper.GetChild(parent, i);
      child = v as T ?? GetVisualChild<T>(v);
      if (child != null)
        break;
    }
    return child;
  }
}