Nitra has a continental climate with hot summers and cold winters. The best time to visit is May through September, when the days are long and warm and outdoor walking around the castle and hills is pleasant. August can be very hot, pushing above 30°C regularly. Spring is particularly good for the countryside around Dražovce church.
The Agrokomplex trade fair grounds on the edge of town host several major agricultural fairs each year, most notably Agrokomplex in August — the largest agricultural fair in Central Europe — which brings crowds to the city but also fills accommodation. If you're not there for the fair, avoid that week.
Winter is quiet and cold, but the castle can be dramatic against a winter sky and the museums are uncrowded.