In north Sweden in September it will get a few degrees Celcius under zero at night. Then slowly the temperatures will go down until they reach to -20 C in December. The coldest months are January and February. Then it is common to get under -30 C for at least a few days to weeks. In March the sun will be strong so then it is not likely it will get under -15 C. The frost will slowly disappear in April.
These are the common temperatures. There are winters it will not get under -30 C, and sometimes above 0 C temperatures in February.