Weather in Rovaniemi is extremely volatile depending on time of the year. In Finland generally are four seasons spring, summer, fall and winter. In winter time there is much snow in Lapland, average lowest temperature at Rovaniemi in January is about – 20 degrees and average highest temperature at Rovaniemi in July is about 20 degrees.
Area of Rovaniemi is arctic zone so weather is mild and subarctic. At the summer time sun won’t go down even at night so it’s bright always at summer, but in winter time sun doesn’t really come up and winter time is mostly dark.
In Lapland there is phenomenon called Aurora Borealis or Northern Lights, it attracts many tourists in Rovaniemi. Aurora Borealis are natural light displays in the sky and they are usually cognizable at night time.
To avoid freezing at winter time, you should have warm clothes and lots of layers and of course water proof and warm outdoor clothes.