Nuremberg is worth visiting at almost any time of year. Summer (June–August) brings warm weather and outdoor dining on the Hauptmarkt, with temperatures typically 20–25°C. The city's museums and the Nazi Rally Grounds are best visited when you have the energy and daylight to do them justice, which rules out December.
The Christkindlmarkt runs from late November through Christmas Eve and transforms the Hauptmarkt into Germany's most famous Christmas market — atmospheric, crowded, and genuinely traditional in a way that many comparable events are not. Book accommodation months in advance for the Christmas season; prices double and rooms disappear quickly.
Spring and autumn are quiet and pleasant. The half-timbered old city is beautiful under autumn light, and the beer gardens in Gostenhof fill up from April onward. Winter outside the Christmas season is cold (often below freezing), grey, and uncrowded — good for museum visits.