The best time to go to Osowiec is during the late spring (May to early June) or early autumn (September to early October) to experience comfortable weather for exploring.
During these months, the temperatures are mild, typically ranging from 15°C to 20°C (59°F to 68°F), making it ideal for outdoor activities like visiting the Osowiec Fortress or exploring the nearby Biebrza National Park. You'll find fewer crowds compared to the peak summer season, allowing for a more relaxed and reflective experience, especially for couples interested in history and nature.
For those interested in birdwatching and wildlife photography, early spring (April) can also be a good choice as migratory birds return to the Biebrza Marshes. While it can be cooler, the opportunity to observe diverse bird species is at its peak.