Summer is the easiest season for Lohja, when Lake Lohjanjarvi becomes part of the visit and the town's apple-and-garden identity makes sense. May to September is best for swimming, paddling, walking and orchard country.
Tytyri Mine Experience and St. Lawrence Church make Lohja workable outside summer too. Winter is quiet and dark, but the underground mine tour and medieval church are not weather-dependent in the same way as the lake shore.