Spring and Autumn is the best time to visit Nagoya. Japan has very hot and humid summer so unless you are already accustomed to sweating heavily then avoid visitng Nagoya in the summer months. It rains very heavily in June-July so avoid those months too. However, if you are looking for cheap deals on hotels and flights then consider coming to Naogya in the off-season. Spring and Autumn are the best times when you can see all the rich colors in display and flowers in full bloom.