Nagoya has a hot, sticky summer (highs of 33°C in August with high humidity) and a relatively dry, cold winter (lows below freezing in January, occasional snow). The shoulder seasons are the rewarding ones: cherry blossom along the Yamazaki River and around Nagoya Castle peaks in early April, and the autumn maples in Tokugawa-en and Tsuruma Park hit in mid- to late November.
Two events worth timing a trip around: the Nagoya Festival in mid-October, which fills Hisaya-odori with samurai parades led by costumed actors playing the three Aichi warlords (Nobunaga, Hideyoshi, Ieyasu); and the World Cosplay Summit in late July, when Osu fills with elaborate costumes from across the globe.