Living in Northern Europe does not sound that you might get sunburned but when is the best time to come to Finland? If you are a person that loves the warmth of summer, come in July or during the first two weeks in August. That is the warmest period of summer. If it does not matter, come in June because most of the Finnish summer holidays start after 24th of June. Feeling more a snow person, do come in late February or early March. This is the period when there is much sunshine and it's not so cold anymore but snow guaranteed. Most of the ski resorts are in best shape and days are longer. If you are a hunting person, come in September or early October - that is the period of the best hunting. Contact http://www.riista.fi/riistaen/ to get all the info you need for hunting in Finland. Being a Santa Claus fan, the best time to come to Finland for you is just before the Christmas. There is Santa Claus World in Rovaniemi, see details here http://www.santaclaus.fi/?deptid=14561
It is recommended you stay a short but definitely the best 3 summer weeks in Finland - do come in the last two weeks in July until the first week of August. You lose the whitest nights but get the best of Finnish summer otherwise. Rent a cottage by the lake and start listening the sound of silence live. See the details about renting a cottage in Finland http://www.finland.com/
Here is the link to the latest weather information from the Finnish Meteorological Institute.