The least expensive time to visit Berlin is the winter.
This is because few tourists come to Berlin during the winter months. It’s an especially good time to visit if you don’t like crowds and you’re looking for some deals on hotels or airfare.
However, there is a reason Berlin sees so few visitors during the winter–it can get extremely cold, with highs in the low 50s and occasional storms.
So if you want to get out and see the city at all, you should try to avoid the winter.
On the other hand, summer is the peak season. There’s no better time for relaxing in an outdoor cafe or beer garden. Temperatures can get up into the 90s, but it usually cools off considerably in the evenings.
Of course, summer is also the most crowded and expensive time to visit Berlin.
For a compromise, visit during the early fall, in September or October. The worst of the crowds will be gone, and it will be easier to find bargains.
The weather won’t be quite as nice, but you won’t have to deal with the worst of the heat, and it should still be warm enough for outdoor activities.
There’s always something to do in Berlin, so deciding when to go mostly depends on your priorities.