Finland is a beautiful country to explore and is less crowded with tourists than other European countries. The Greater Helsinki area is the most northern area in the world that has a million or more people. With that comes coldness and darkness. Bring layers regardless of when you go, and we'd definitely recommend going in the summer. During the winters, the sun never comes up, so even the spring and fall are going to have short days. Best to go during the summer when you can enjoy the daylight.
Both Helsinki and Turku have Wifi hotspots around the city, so it's quite easy to stay off of mobile. We'd still recommend having a least one person in your travel group have an international plan set up on their phone.