Use following code :
private void Calendar1_SelectionChanged (object sender,
System.EventArgs e)
{
SelectedDatesCollection theDates = Calendar1.SelectedDates;
TimeSpan timeSpan = theDates[theDates.Count-1] - theDates[0];
TextBox1.Text = String.Format("You have selected {0} day(s).", timeSpan.Days + 1);
}