Hi Ebrahim,

Thank you for the provided images.

By default, RadCalendar allows you to select a single date by clicking on it. Then, clicking a selected cell will de-select it. The SelectedDate property does not support null values so when there is no selected date, the default value will be the minimum value. In order to prevent selecting specific dates, you can cancel the RadCalendar.SelectionChanging event. In the event handler, you can check if the NewDates collection is empty. If yes, cancel the event. In the SelectionChanged event handler, you can update the Label.

public RadForm1()
{
    InitializeComponent();
    this.radCalendar1.SelectionChanged += RadCalendar1_SelectionChanged;
    this.radCalendar1.SelectionChanging += RadCalendar1_SelectionChanging; ;
}

private void RadCalendar1_SelectionChanging(object sender, SelectionEventArgs e)
{
    if(e.NewDates.Count== 0)
    {
        e.Cancel = true;
    }
}

private void RadCalendar1_SelectionChanged(object sender, EventArgs e)
{
    label1.Text = radCalendar1.SelectedDate.ToString();
}

Additional information about the calendar's selection is available in the Selecting Dates help article.

I hope this information helps. 

Regards,
Dinko
Progress Telerik