Selected items of listview checkbox in MVVM WPF

Question

 
I want to pass selected items of listview checkbox in view model, later I will use further process to store in database.
Code in the FormWeek.xaml as

<Window.DataContext>
<Binding Source="{StaticResource Locator}" Path="TaskExecDefModel">
</Binding>
</Window.DataContext>
<Window.Resources>
<ResourceDictionary>
<DataTemplate x:Key="ItemDataTemplate">
<CheckBox x:Name="checkbox" Content="{Binding}" Command="{Binding CheckBoxCommand}" CommandParameter="{Binding ElementName=checkedListView, Path=SelectedItems}" IsChecked="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListViewItem}}, Path=IsSelected}" />
</DataTemplate>
</ResourceDictionary>
<StackPanel Grid.Column="1" Grid.Row="3" Margin="5">
<CheckBox x:Name="selectAll" Content="Select all" Click="OnSelectAllChanged"/>
<ListView x:Name="checkedListView" SelectionMode="Multiple" ItemsSource="{Binding CollectionOfDays}" DataContext="{Binding}" ItemTemplate="{StaticResource ItemDataTemplate}" SelectedValue="WorkingDay" CheckBox.Unchecked="OnUncheckItem" SelectionChanged="SelectDays" SelectedItem="{Binding SelectedItems}">
</ListView>
</StackPanel>

Code in FormWeek.xaml.cs

private void SelectDays(object sender, SelectionChangedEventArgs e)
{
(this.DataContext as TaskExecDefinitionViewModel).OnCheckBoxCommand(checkedListView.SelectedItems,true);
}

My View Model TaskWeek.cs as follows

//Declarationprivate RelayCommand<object> _checkBoxCommand;
public ObservableCollection<string> CollectionOfDays
{ get; set; }
public ObservableCollection<string> SelectedItems
{ get; set; }
public RelayCommand<object> CheckBoxCommand
{
get
{
if (_checkBoxCommand == null)
{
_checkBoxCommand = new RelayCommand<object>((args) => OnCheckBoxCommand(args,true));
// _checkBoxCommand = new RelayCommand(() => OnCheckBoxCommand(object args));
} return _checkBoxCommand;
}
}
//ConstructorCollectionOfDays = new ObservableCollection<string>();
//Methodprivate void GetWeekDays()
{
try
{
Application.Current.Dispatcher.BeginInvoke(DispatcherPriority.Background, new Action(() => { CollectionOfDays.Add("Saturday");
CollectionOfDays.Add("Sunday");
CollectionOfDays.Add("Monday");
CollectionOfDays.Add("Tuesday");
CollectionOfDays.Add("Wednesday");
CollectionOfDays.Add("Thursday");
CollectionOfDays.Add("Friday"); }));
}
catch(Exception Ex)
{
MessageBox.Show(Ex.Message, "TaskWeek:GetWeekDays");
}
}
public void OnCheckBoxCommand(object obj, bool _direction)
{
try
{
if (SelectedItems == null)
SelectedItems = new ObservableCollection<string>();
if (obj != null)
{
SelectedItems.Clear();
StringBuilder items = new StringBuilder();
if (_direction)
{
foreach (string item in CollectionOfDays)
{
items.AppendFormat(item + ",");
}
}
MessageBox.Show(items.ToString());
}
else
SelectedItems.Clear();
}
catch (Exception ex)
{ MessageBox.Show(ex.Message, "TaskDefinition:OnCheckBoxCommand");
}
}

And below is button click commond to save the data.

<Button Grid.Column="2" Grid.Row="2" Content="Save" HorizontalAlignment="Center" VerticalAlignment="Bottom" Command="{Binding InsertExecDefCommand}" Margin="5" >

Now my requirement is to pass selected listview items to view model through command object. I had done this using following code in FormWeek.xam.cs through SelectionChanged event as

private void OnSelectedItems(object sender, RoutedEventArgs e)
{
StringBuilder items = new StringBuilder();
foreach (string item in checkedListView.SelectedItems)
{
items.AppendFormat(item + ",");
}
string AllDays= items.ToString();
}

But please let me know how to achieve this logic through MVVM. How to get selecteditems in my view model TaskWeek.cs

After R&D and google i had done chages through RelayCommand. In OnCheckBoxCommand method foreach statement is wrong it is passing all days. I want to pass only selected listview item. Please suggest me what is wrong in OnCheckBoxCommand method.