If I have a logger that is calibrated to 25C/60%RH can I still use it if I need to see temperature of a freezer -20C

Yes, you can use it at that temperature. The calibration points are just the reference points we use to validate the accuracy. They don’t have any effect on the overall accuracy of the device. Our only word of caution for using this logger at -20°c would be that you will likely see the battery life drop considerably. Typically we recommend using a logger with an external sensor for this type of application.