Expander Control in Wpf

Use the Expanded event:

<Expander Expanded="Expander_Expanded" />

and then call the command from the code behind:

private void Expander_Expanded(object sender, RoutedEventArgs e)
{
    ViewModel vm = (ViewModel)DataContext;
    if (vm.AdvancedCommand.CanExecute(vm.gridAdvances))
        vm.AdvancedCommand.Execute(vm.gridAdvances));
}

Change ViewModel to whatever type your DataContext is.

Add a reference to System.Windows.Interactivity.dll (Project->Add Reference->Assemblies->Extensions in Visual Studio) and add an interaction trigger to the Expander:

<Expander xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="IsExpanded" >
            <i:InvokeCommandAction Command="{Binding AdvancedCommand}" CommandParameter="{Binding ElementName=gridAdvances}" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</Expander>

I suggest using the solution of mm8 with the event Expanded suggested by Freggar

<UserControl
xmlns:i="clr-amespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"/>
   <Expander>
       <i:Interaction.Triggers>
           <i:EventTrigger EventName="Expanded" >
              <i:InvokeCommandAction Command="{Binding AdvancedCommand}" CommandParameter="{Binding ElementName=gridAdvances}" />
           </i:EventTrigger>
       </i:Interaction.Triggers>
   </Expander>
<UserControl>

I had the same problem, for some reason it is not possible to do simple an event trigger on attached property in the TreeView object (.net core 3 + wpf).

<i:Interaction.Triggers>
  <i:EventTrigger EventName="Expander.Expand">
    <i:InvokeCommandAction Command="{Binding MyCommand}"/>
  </i:EventTrigger>
</i:Interaction.Triggers>

I had to solve it via custom behavior class.See below for inspiration ;)

public class TreeViewSelectionBehavior : Behavior<TreeView>
{
    public object SelectedItem
    {
        get { return (object)GetValue(SelectedItemProperty); }
        set { SetValue(SelectedItemProperty, value); }
    }

    public ICommand Command
    {
        get { return (ICommand)GetValue(CommandProperty); }
        set { SetValue(CommandProperty, value); }
    }

    public static readonly DependencyProperty SelectedItemProperty =
        DependencyProperty.Register("SelectedItem", typeof(object), typeof(TreeViewSelectionBehavior), new UIPropertyMetadata(null, OnSelectedItemChanged));

    public static readonly DependencyProperty CommandProperty = 
        DependencyProperty.Register("Command", typeof(ICommand), typeof(TreeViewSelectionBehavior), null);

    private static void OnSelectedItemChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
    {
        var item = e.NewValue as TreeViewItem;
        if (item != null)
        {
            item.SetValue(TreeViewItem.IsSelectedProperty, true);
        }
    }

    protected override void OnAttached()
    {
        base.OnAttached();

        this.AssociatedObject.SelectedItemChanged += OnTreeViewSelectedItemChanged;

        this.AssociatedObject.AddHandler(TreeViewItem.ExpandedEvent, new RoutedEventHandler(this.Expanded));
    }

    private void Expanded(object sender, RoutedEventArgs e)
    {
        TreeViewItem item = e.OriginalSource as TreeViewItem;

        if (this.Command.CanExecute(item))
        {
            this.Command.Execute(item);
        }
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (this.AssociatedObject != null)
        {
            this.AssociatedObject.SelectedItemChanged -= OnTreeViewSelectedItemChanged;
            this.AssociatedObject.RemoveHandler(TreeViewItem.ExpandedEvent, new RoutedEventHandler(this.Expanded));
        }
    }

    private void OnTreeViewSelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
    {
        this.SelectedItem = e.NewValue;
    }
}

Then usage is simple

<i:Interaction.Behaviors>
  <local:TreeViewSelectionBehavior SelectedItem="{Binding SelectedItem}" Command="{Binding ExpandHistoricalItemCommand}"/>
</i:Interaction.Behaviors>