Even when it's freezing you can use your kamado!
But make sure that you let the temperature rise as slowly as possible, especially if it is cold and/or damp weather. This allows the ceramic to settle and prevents cracks and/or cracking of the ceramic.
To allow the temperature to rise slowly, start with the top and bottom vents that are only open for a small part. You can slowly open them further and further to let the temperature rise. So be patient.
Do you use a rain cover? Great! This protects your kamado against rain and dirt, but not against frost.