Peak season: (Jul & Aug) There are a lot of tourists who visit Germany during this period of the year. The weather conditions are good if you want to explore different cities.
Shoulder season: (Apr–Jun, Sep–Oct) Not much crowded during these months, the prices are lower compared to the ones in the peak season. The weather is sunny most of the times, ideal for outdoor activities.
Off-season: (Nov-Mar) There is severe snowfall in most of the cities and the conditions are pretty cold. Travelling may get difficult during this period.