Mantua is at its best in spring (April–June) and early autumn (September–October). The city sits in the Po plain, which means hot, humid summers and cold, foggy winters. July and August are busy with Italian and international tourists, and the heat can make the dark interiors of the palaces feel like a relief.
September is particularly good: the Festivaletteratura, held over five days in early September, brings authors from across the world and takes over public spaces around the city for readings, performances, and conversations. The festival is worth timing a visit around even if you don't speak Italian — many events are in English, and the atmosphere is lively.
The lotus flowers on Lago Superiore bloom in July and August; if you want to see them, this is the one reason to come in summer. Winters are cold, foggy, and quiet — not unpleasant if you want the palaces to yourself, but the mist that makes Mantua atmospheric can also make it grey.