# QCalendarWidget Class
class Calendar(QCalendarWidget):

    # constructor
    def __init__(self, parent=None):
        super(Calendar, self).__init__(parent)

    # overriding the customEvent method
    def customEvent(self, event):

        print("Custom Event")

# importing libraries
from PyQt5.QtWidgets import * 
from PyQt5 import QtCore, QtGui
from PyQt5.QtGui import * 
from PyQt5.QtCore import * 
import sys

# QCalendarWidget Class
class Calendar(QCalendarWidget):

    # constructor
    def __init__(self, parent = None):
        super(Calendar, self).__init__(parent)

    # overriding the customEvent method
    def customEvent(self, event):

        print("Custom Event")




class Window(QMainWindow):

    def __init__(self):
        super().__init__()

        # setting title
        self.setWindowTitle("Python ")

        # setting geometry
        self.setGeometry(100, 100, 650, 400)

        # calling method
        self.UiComponents()

        # showing all the widgets
        self.show()

    # method for components
    def UiComponents(self):

        # creating a QCalendarWidget object
        # as Calendar class inherits QCalendarWidget
        self.calendar = Calendar(self)

        # setting geometry to the calendar
        self.calendar.setGeometry(50, 10, 400, 250)

        # setting cursor
        self.calendar.setCursor(Qt.PointingHandCursor)

        # close the calendar
        self.calendar.selectionChanged.connect(lambda : self.calendar.customEvent("Change"))




# create pyqt5 app
App = QApplication(sys.argv)

# create the instance of our Window
window = Window()

# start the app
sys.exit(App.exec())

Custom Event
Custom Event
Custom Event
Custom Event