public RadForm1()
 {
     InitializeComponent();

     this.radCalendar1.AllowMultipleSelect = true;
     this.radCalendar1.SelectionChanging += this.RadCalendar1_SelectionChanging;
     this.radCalendar1.MouseDown += this.RadCalendar1_MouseDown;
     this.radCalendar1.MouseUp += this.RadCalendar1_MouseUp;
 }

 CalendarCellElement cellSelectionStart;
 bool isMouseDown;
 private void RadCalendar1_MouseUp(object sender, MouseEventArgs e)
 {
     this.cellSelectionStart = null;
     this.isMouseDown = false;
 }

 private void RadCalendar1_MouseDown(object sender, MouseEventArgs e)
 {
     CalendarCellElement cell = this.radCalendar1.ElementTree.GetElementAtPoint<CalendarCellElement>(e.Location);
     if (cell != null)
     {
         this.cellSelectionStart = cell;
         this.isMouseDown = true;
     }
 }
 private void RadCalendar1_SelectionChanging(object sender, SelectionEventArgs e)
 {
     if (this.isMouseDown && e.NewDates.Count > 0)
     {

         var dates = e.NewDates.Where(d => d.Date != this.cellSelectionStart.Date);
         if (dates.Count() > 0)
         {
             e.Cancel = true;
         }
     }
 }