Bindings not updating inside datagrid

Question

I have a datagrid where I change a cell depending on what is chosen from the other column.

Let's say, my first column shows Weekdays (enums). If the user choose "Monday", the cell in the second column will be a TextBox. If the user chosoe a different day(eg. Friday), it'll become a Textblock with Text="Hooray!", else Textblock Text="".

Both Textbox and Textblock binds in the same property. But this binding does not work. Please help...

XAML

<DataGrid Margin="0,10,0,0"
                          x:Name="dataGrid"
                          AutoGenerateColumns="False"
                          IsSynchronizedWithCurrentItem="True"
                          ItemsSource="{Binding TheCollection}"
                          SelectedItem="{Binding TheSelectedItemFromTheCollection}">
                    <DataGrid.Columns>
                        <DataGridTemplateColumn Header="Days"
                                                CanUserReorder="False"
                                                CanUserResize="False"
                                                CellTemplate="{StaticResource local.DataGridDays}" />
                        <DataGridTemplateColumn Header="Value"
                                                CanUserReorder="False"
                                                CanUserResize="False"
                                                CellTemplate="{StaticResource local.DataGridValue}" />
                    </DataGrid.Columns>
</DataGrid>

<DataTemplate x:Key="local.DataGridValuesEditable">
    <StackPanel>
        <Border Background="White"
                BorderThickness="1"
                Padding="4">
            <TextBox Text="{Binding InitialValue, RelativeSource={RelativeSource Self}, UpdateSourceTrigger=PropertyChanged}"
                     VerticalAlignment="Bottom"
                     Margin="0" />
        </Border>
    </StackPanel>
</DataTemplate>
<DataTemplate x:Key="local.DataGridValuesDefault">
    <StackPanel>
        <Border Background="White"
                BorderThickness="1"
                Height="30"
                Padding="4">
            <TextBlock  Text="{Binding InitialValue, Mode=TwoWay, RelativeSource={RelativeSource Self}, UpdateSourceTrigger=PropertyChanged}"
                        VerticalAlignment="Bottom"
                        Margin="0"
                        Width="55" />
        </Border>
    </StackPanel>
</DataTemplate>
<DataTemplate x:Key="local.DataGridValue">
    <ContentControl>
        <ContentControl.Style>
            <Style TargetType="ContentControl">
                <Setter Property="ContentTemplate"
                        Value="{DynamicResource local.DataGridValuesDefault}" />
                <Style.Triggers>
                    <DataTrigger Binding="{Binding SelectedDay, UpdateSourceTrigger=PropertyChanged}"
                                 Value="Monday">
                        <Setter Property="ContentTemplate"
                                Value="{DynamicResource local.DataGridValuesEditable}" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </ContentControl.Style>
    </ContentControl>
</DataTemplate>

ViewModel

private ObservableCollection<TheModel> _theCollection;

public ObservableCollection<TheModel> TheCollection
{
    get
    {
        if (_theCollection == null)
            _theCollection = new ObservableCollection<TheModel>();
        return _theCollection;
    }
    set
    {
        _theCollection = value;
    }
}

private TheModel _theSelectedItemFromTheCollection;

public TheModel TheSelectedItemFromTheCollection
{
    get
    {
        if (_theSelectedItemFromTheCollection == null)
            _theSelectedItemFromTheCollection = new TheModel();
        return _theSelectedItemFromTheCollection;
    }
    set
    {
        _theSelectedItemFromTheCollection = value;
        NotifyPropertyChanged("TheSelectedItemFromTheCollection");
    }
}

Model

public string SelectedDay
{
    get { return day; }
    set
    {
        day = value;
        if (day == TheWeekDays.Friday.ToString())
        {
            InitialValue = "Hooray!!";
        }
        else
            InitialValue = string.Empty;        
        NotifyPropertyChanged("SelectedDay");
    }
}

private string _initialValue;
public string InitialValue
{
    get { return _initialValue; }
    set
    {
        _initialValue = value;
        NotifyPropertyChanged("InitialValue");
    }
}

public IEnumerable<WeekDays> TheWeekDays
{
    get
    {
        return Enum.GetValues(typeof(WeekDays))
                    .Cast<WeekDays>();
    }
}

Answer 1

The following is the desired situation:

The DataContext of local.DataGridValuesDefault and local.DataGridValuesEditable should be the row item TheModel.

With this DataContext, the bindings can be written as follows:

<TextBox Text="{Binding InitialValue,UpdateSourceTrigger=PropertyChanged}"

and

<TextBlock Text="{Binding InitialValue}"

The DataContext of local.DataGridValue contains the desired item. A ContentControl will turn its Content into the DataContext of its ContentTemplate, so the Content needs to be linked with the outer DataContext. Also, the trigger binding can be simplified:

<DataTemplate x:Key="local.DataGridValue">
    <ContentControl Content="{Binding}">
        <ContentControl.Style>
            <Style TargetType="ContentControl">
                <Setter Property="ContentTemplate"
                        Value="{DynamicResource local.DataGridValuesDefault}" />
                <Style.Triggers>
                    <DataTrigger Binding="{Binding SelectedDay}"
                                 Value="Monday">
                        <Setter Property="ContentTemplate"
                                Value="{DynamicResource local.DataGridValuesEditable}" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </ContentControl.Style>
    </ContentControl>
</DataTemplate>