Dusseldorf is one of those places you can visit all the year through. Not having an ideal weather, the winters can get very cold and the summers are known to sport the occasional shower. That being said, the months of July and August see an influx of tourists from all over. The summer temperatures are cool due to the rain; carry an umbrella whenever you step out to avoid any surprise.
Some of the carnivals take place in the months of July or November, so this is also a good time to visit this place.