Mannheim is situated in one of the warmer regions of Germany in the rhine valley. The summer high temperatures are around 30 degree Celsius, at night from June to August around 20 degree. The winters are mild and most time over zero degree. Not much snow. In spring and autumn there are more rainy days, especially april/may and novembre. So the best times to visit Mannheim would be spring or summer, but every season has its highlights,  where you could go.

Here is the link to Mannheim Weather - Five Day Forecast.