It is colder than almost any visitor has experienced, and it matters less than they expect.
Daytime temperatures run about minus ten to minus twenty in December, near minus thirty at the coldest in January, minus fifteen to minus twenty in February and minus five to minus fifteen by March. Add wind chill on a moving sled and the number on the sled is lower than the number on the thermometer.
What makes it manageable is that Finnish operators have been dealing with this for a very long time. Every safari provides a full thermal overall, insulated boots, mitts and usually a balaclava, and the tours where people are miserable are almost always the ones where somebody wore jeans and a cotton hoodie underneath.
Below about minus thirty-five most operators stop running, for the dogs as much as for the visitors, and they refund. It is uncommon but it happens, which is one more reason to have a flexible day in the itinerary rather than booking the safari for the afternoon before your flight.