Nizhny Novgorod is worth visiting in any season, but the most pleasant weather is from May through September. July and August are warm and the rivers are at their most appealing; the city's parks and embankments fill with people in the evenings. Summer river cruises from Moscow often include a stop here.
Winter is cold — January temperatures regularly drop to -15°C — but the city looks good under snow, and the Russian Orthodox Christmas and New Year celebrations are worth experiencing if you can handle the cold. The Kremlin walls and Chkalov Stairs in winter have a particular atmosphere. Spring (April–May) brings mud and then sudden blooming; autumn (September–October) is crisp and often sunny.
The city is less crowded than Moscow at any time of year, so there is no particular high season to avoid.