April through October is the best window for visiting Gniezno — the old town is pleasant to walk, the cafes put out their terrace furniture, and the cathedral grounds feel alive. June is particularly good: the Lech Fest brings a medieval festival atmosphere to the area around Gniezno Cathedral, with costumed events and craft markets that suit the city's historical character well.
Pilgrims tend to converge in numbers around two key dates: April 23, the feast of Saint Adalbert (Poland's patron), and May 3, the Polish national holiday. If you want the cathedral to yourself, avoid those weekends. Winters are cold and grey, but the cathedral is heated, uncrowded, and arguably more atmospheric when you have it largely to yourself. The bronze doors are equally impressive whether it's June or January.