Calibration refers to the process of flushing the display with a single colour to prevent 'ghosting' (an effect specific to E-Paper displays where the remnants of the previous image can be seen on the current one). It takes several minutes to finish the calibration(around 10 mins for the 2-colour displays and around 20 mins for the 3-colour displays) so please choose hours where you are less likely to need the display. It is recommended to calibrate at least thrice a day.