<script runat="server">
Sub DaySelect(obj As Object, e As DayRenderEventArgs)
  If e.Day.IsSelected Then
    If e.Day.IsOtherMonth = "TRUE" Then
      Label1.Text = "NO"
    Else
      Label1.Text = "YES"
    End If
  End If
End Subv </script>

<form runat="server">
  <asp:Calendar id="cal1" runat="server"
  OnDayRender="DaySelect"/>
  The selected date is in the current month:
  <asp:Label id="Label1" runat="server"/>
</form>