Lausanne is a year-round city — the museums, restaurants, and urban life carry on regardless of season. That said, the lakefront transforms in summer (June to September), when the promenade at Ouchy fills with people, the outdoor cafes stay open late, and the Alps across the water are snowless and clear in the heat.
Spring and autumn are ideal for the Lavaux vineyard walk: the light is better, the crowds are smaller, and in October the vines turn gold. Winter is cold and grey, but the Christmas market in the old town is genuine rather than staged, and the city never empties the way smaller Swiss towns can.
July brings the Lausanne Estival jazz and music festival, with free outdoor concerts across the city. The Paléo Festival in nearby Nyon (July) is one of Switzerland's biggest open-air concerts and draws crowds to the whole region.