Code

public class CalendarCommands
{
	static public ICommand GetItemModifyingCommand(DependencyObject obj)
	{
		return (ICommand)obj.GetValue(ItemModifyingCommandProperty);
	}

	static public void SetItemModifyingCommand(DependencyObject obj, ICommand value)
	{
		obj.SetValue(ItemModifyingCommandProperty, value);
	}

	public static DependencyProperty ItemModifyingCommandProperty =
		DependencyProperty.RegisterAttached(
			"ItemModifyingCommand",
			typeof (ICommand),
			typeof (CalendarCommands),
			new PropertyMetadata(CalendarChanged));

	static void CalendarChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
	{
		var calendar = (Calendar)d;

		calendar.ItemModifying += (s, args) =>
		{
			var cmd = GetItemModifyingCommand(d);
			if (cmd != null)
				cmd.Execute(args);
		};
	}
} 

Code

<planner:Calendar Name="calendar"
    local:CalendarCommands.ItemModifyingCommand=
        "{Binding MyCommand, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=local:Window1}}" 

Code

public class MyCommand : ICommand
{
	public void Execute(object parameter)
	{
		var args = (ItemModifyConfirmEventArgs)parameter;
		Debug.WriteLine(args.Item.HeaderText);
	}
	...
}