Most airlines will offer Germany-wide sales, i.e., they offer similar fares to anyplace in Germany, or have only slight variations. Lufthansa, Air France and Alitalia often have the best fares. Spring or autumn are usually wonderful in Berlin, and the place is so far north that in June the days are very long (earlier this month I was in Berlin, when dawn was at 9 am and dusk at 4 pm!). The best fares are usually in February or March, though, when I've also experienced wonderful weather.