Oktoberfest

for a whole month, the city is running with beer of all sorts, and this is a good time to try the different types from all the different regions of Germany. The festival originated in Munich and is held in a much larger scale there but Berlin still attracts thousands of visitors during this time. 

More Festivals in Berlin
Christmas markets
Christopher Street Day
Festival of Lights
ITB
Karneval der Kulturen
Lange Nacht der Museen