Code Behind

private void Grid_Hold(object sender, System.Windows.Input.GestureEventArgs e)
{
    Grid g = (Grid)sender;
    // This throws an error because you can't pass a Grid as a Control in the first parameter
    VisualStateManager.GoToState(g, "Context", true);
}

ItemTemplate

<DataTemplate x:Key="ItemTemplate1">
            <Grid Hold="Grid_Hold" >
                <VisualStateManager.VisualStateGroups>
                    <VisualStateGroup x:Name="CommonStates">
                        <VisualState x:Name="Normal">
                            <Storyboard>
                                <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="ContextGrid">
                                    <DiscreteObjectKeyFrame KeyTime="0" Value="Collapsed"/>
                                </ObjectAnimationUsingKeyFrames>
                            </Storyboard>
                        </VisualState>
                        <VisualState x:Name="Context">
                            <Storyboard>
                                <ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="Visibility" Storyboard.TargetName="ContextGrid">
                                    <DiscreteObjectKeyFrame KeyTime="0" Value="Visible"/>
                                </ObjectAnimationUsingKeyFrames>
                            </Storyboard>
                        </VisualState>
                    </VisualStateGroup>
                </VisualStateManager.VisualStateGroups>
                <!-- Main Content -->

                <Grid x:Name="ContextGrid" Background="#99000000" Visibility="Collapsed">
                   <!-- Context Content -->
                </Grid>
                
            </Grid>
        </DataTemplate>