Lake Garda is crowded in July and August, and Lazise — being a pretty walled town with good access from the motorway — receives its share. Spring (April to June) and early autumn (September to October) are the best times: the weather is warm, the lake is swimmable, and the streets are not impossible to walk. Bardolino wine harvest in October is worth timing a visit around.
Winter is quiet but not dead — the Christmas markets at Lazise run from November through January and draw visitors from across northern Italy.