June to August is the obvious season — long days (the sun rises before 4am and sets after 10pm), warm enough for lake swimming, festivals and outdoor concerts. The Sibelius Festival in early September is a draw for classical music fans.
Early March brings the Lahti Ski Games (the FIS Nordic World Cup ski-jumping and cross-country skiing) — the largest annual sports event in the city, and the best time to see the ski jumps in action. Winter (December to early March) is cold (well below zero, often below –15°C) but Lahti's snow is reliable and the city is a good base for cross-country skiing.
April, May and October are quieter and cheaper; the November–early March period can feel grim if there is no snow on the ground.