April through October is the main season — the lake is swimmable from June, the lakeside promenade is at its best in spring when it's lined with flowering plants, and the general atmosphere of outdoor life is in full swing. July is the time for the Lugano Estival Jazz, a free jazz festival held on and around Piazza della Riforma that draws large crowds and fills the city's hotels. Summer weekends bring significant numbers of day-trippers from Milan (just over an hour away by train), which makes the lakefront busy and some restaurants fully booked.
Spring is arguably the most pleasant time to visit: warm enough to sit outside, without the midsummer crowds, and the blossom along the lakeside is genuinely worth seeing. Winter is quieter and mild by Swiss standards, and Monte Brè in particular retains sunshine when the valley is grey. Christmas sees some festive decoration in the old town, but Lugano does not transform itself for winter tourism the way the ski resorts do.