Try this code
final DecoratedPopupPanel appointmentPopup = new DecoratedPopupPanel(true);
appointmentPopup.setAutoHideEnabled(true);
calendar.addMouseOverHandler(new MouseOverHandler<Appointment>(){
public void onMouseOver(MouseOverEvent<Appointment> event) {
Element element = (Element)event.getElement();
String str ="<br>"+event.getTarget().getTitle()+"</br>"+event.getTarget().getDescription();
appointmentPopup.setWidget(new HTML(""+str));//event.getTarget().getTitle()));
//appointmentPopup.setWidget(new HTML(event.getTarget().getDescription()));
int left = element.getAbsoluteLeft() + 10;
int top = element.getAbsoluteTop() + 10;
appointmentPopup.setPopupPosition(left, top);
appointmentPopup.show();
}
});