The Mazurian Lakes have a very distinct season driven by sailing and swimming.
June to August is the main tourist season. The lakes are warm enough to swim (18–22°C in July), the daylight lasts until 9pm, and the marinas fill with boats. July and August are the peak months with school holidays bringing large numbers of Polish families; accommodation in the popular towns (Mikołajki, Giżycko) must be booked months in advance for August.
June is excellent if you can: fewer people than July/August, the weather is warming, and the wildflowers are at their best in the meadows.
September is one of the best months for kayaking and cycling: the summer crowds have thinned, the forests are beginning to turn gold and rust, mushroom season starts, and the weather is often settled and warm. Sailing continues on the lakes. Some facilities begin to close.
October to May is effectively off-season. Most lake-oriented businesses close after September. However, the forests are beautiful in autumn, the ice fishing season on frozen lakes runs from January, and Wolf's Lair is far better appreciated without the summer tour groups.
Mosquitoes are a serious consideration in June and July, especially near reed beds and in forested lakeshores. Bring repellent; it is not optional.