The Finnish sauna is a substantial part of Finnish culture.
“There is nothing that Finns have been so unanimous about as their sauna. This unanimity has remained unbroken for centuries and is sure to continue as long as there are children born in their native land, as long as the invitation still comes from the porch threshold in the evening twilight: “The sauna is ready.”
Maila Talvio 1871-1951
I don't know the exact figures but I'm pretty sure that today more than 90% of the Finnish apartments and houses have a sauna. Either it's private inside the flat or shared with the housing cooperative where each resident have their own weekly sauna turn. It is estimated that there are two million saunas in Finland (our population is 5,3 million.)
Shortly put sauna is a small wooden room which is heated into 80-100 celcius degrees with the stove, called kiuas. The stove is heated usually with wood or electricity. The top of the stove is covered with a thick layer of natural stones, which radiate the heat to the room. Then small doses of water is ladled into the kiuas which makes sauna very humid.
Usually when we go to sauna, we do it with the family so yes, we do it naked. But when we gather together with friends, usually we take women's and men's turns separately. Of course some parties have mixed turns. Nakedness is extremely natural for us but of course we value our guests' and there's absolutely no requirement to bathe without a towel or swim suit.
Unlike our friends in other parts of Europe, we don't have any particular traditions in sauna. Some like to sit quietly and some like to have deep conversations. And yes, cold beer tastes extremely good in sauna so naturally some loud laughing or singing can be heard also.
Hale - The wool sock lady who doesn't know how to knit