Вы должны наследовать от QCalendarWidget и перезаписать метод paintCell:
QCalendarWidget.paintCell (художник, прямоугольник, дата)
Параметры:
художник - QtGui.QPainter
rect - QtCore.QRect
date - QtCore.QDate
Извините, я использую PyQt5
import sys
from PyQt5.QtCore import Qt, QRectF
from PyQt5.QtGui import QPainter
from PyQt5.QtWidgets import QCalendarWidget, QApplication
class CalendarWidget(QCalendarWidget):
def paintCell(self, painter, rect, date):
painter.setRenderHint(QPainter.Antialiasing, True)
if (date.day() == 1) or (date.day() == 15):
painter.save()
painter.drawRect(rect)
painter.setPen(Qt.blue)
painter.drawText(QRectF(rect), Qt.TextSingleLine|Qt.AlignCenter, str(date.day()))
painter.restore()
else:
QCalendarWidget.paintCell(self, painter, rect, date)
if __name__ == '__main__':
app = QApplication(sys.argv)
w = CalendarWidget()
w.show()
sys.exit(app.exec_())