For Sweden, our general recommendation is to use sunscreen from March to October.
The reason why we only recommend this period is because the UV index is low (0-2) during the darkest months, November to February, and then it is safe to be out in the sun all day without sunscreen. The UV index is a measure of the strength of the sun's harmful radiation. A high UV index means that you should protect yourself from the sun to reduce the risk of damage and in Sweden the UV index is between 0-8 and it is at its highest in the middle of the day during the summer months. However, it is important to remember to use sunscreen if you go on a skiing holiday, even during the darkest months, when the sun's rays are reflected by the snow.