<!DOCTYPE html>
<html>
<head>
<script  runat="server">
Sub Change(obj As Object, e As EventArgs)
      Dim i As Integer
      For i = 0 To Cal1.SelectedDates.Count - 1
           Response.Write(Cal1.SelectedDates(i).ToShortDateString())
           Response.Write("<br>")
      Next i
End Sub
</script>

</head>
<body>

<form runat="server">
<asp:Calendar ID="Cal1" runat="server" SelectionMode="DayWeekMonth" OnSelectionChanged="Change" />
</form>
</body>
</html>