I found a great fare for Berlin in February for the Berlin Film Festival – Berlinale. 15th- 25th February 2018
Berlin Film Festival has become probably Europe’s best-respected film festival. Awards handed out include the incredibly obscure Panorama Award, the new Generation Crystal Bear and the cute Teddy [bear] Award for LGBT.
In Ryanair website is cheaper than Momondo directly so go ahead to check different dates.
Prepare in advance as it will be expensive in accommodation, so check Airbnb too. You would better stay in the C area of the city and have a transportation plan or a Berlin card for the number of days you plan to visit. This would be the most affordable version.