An image forming apparatus which is capable of increasing printing efficiency by simplifying the calibration operation in time zones during which the frequency of use of the apparatus is high. An image forming apparatus has a plurality of calibration modes set in advance on a time zone-by-time zone basis for operating days. A printer controller determines a day of week and time of day, and selects one of the calibration modes associated with the day of week and time of day. The controller performs cumulative counting of sheets color-printed after a preceding calibration, and determines whether or not calibration in the selected calibration mode should be executed, from a cumulative count value of color-printed sheets and the selected calibration mode. The calibration in the selected calibration mode is executed in response to the determination that the calibration should be executed.