There is nothing more traditional in Finland than a lakeside Sauna right? Well researchers for the Central Finland in Jyväskylä who prepared an exhibition on the sauna have identified the lakeside sauna as a relatively modern trend beginning with urban Finns starting it when they bought second homes beside the lakes.
In the countryside, the family sauna was usually to be found close to the farmhouse and close to the well. But, as increasing numbers of urban residents began acquiring villas and cottages as summer holiday retreats during the latter half of the 19th century, saunas began appearing on lakeshores….
It was during the war period that lakeside saunas became much more widespread. Soon, they began to take over from the bathhouses of the genteel folk, becoming so common that few people today even consider that they have not always been a central feature, and a delight, of the Finnish way of life. – YLE article
This is all consistent with the Sauna moving from being an integral part of life to a leisure activity.