C++ (Qt)
#ifndef CALENDAR_H
#define CALENDAR_H
#include <QCalendarWidget>
#include <QMessageBox>
#include <QEvent>
#include <QMouseEvent>
#include <QTimer>
 
class calendar: public  QCalendarWidget
{
     Q_OBJECT
public:
    explicit calendar(QWidget *parent = 0);
 
private slots:
   void ClickDate(QDate);
  //  void a();
 
protected:
   bool event(QEvent *event);
  // bool b;
};
 
#endif // CALENDAR_H
 

C++ (Qt)
#include "calendar.h"
 
calendar::calendar(QWidget *parent):QCalendarWidget(parent)
{
    //QObject::connect( this,SIGNAL(clicked(QDate)), this , SLOT(ClickDate(QDate))    );
 
 
 
}
 
 
 
//void calendar::a()
//{
//    b = false;
//}
 
void calendar::ClickDate(QDate date)
{
    QMessageBox::about(0,"","a");
//    if (!b){
//        QTimer::singleShot(5,this,SLOT(a()));
//    } else close();
}
 
 
 
bool calendar::event(QEvent *event)
{
    if (event->type()  == QEvent::MouseButtonDblClick)
    {
        QMessageBox::about(0,"","Двойной щелчек");
    }
    return QCalendarWidget::event(event);
}