uses CommCtrl; 
 
procedure TForm1.Button1Click(Sender: TObject); 
var Style: DWord; 
begin 
  Style := GetWindowLong(MonthCalendar1.handle, GWL_STYLE); 
  SetWindowLong(MonthCalendar1.handle, GWL_STYLE, Style 
    or MCS_NOTODAY or MCS_NOTODAYCIRCLE); 
end;