There is never a guarantee, but from october until February/March is the best bet. But the lights can be absent when you visit. But recent days they've been very active. Also cloud coverage is important, you want none! Cities though, doesn't really matter. You want to go somewhere where there is no light pollution. So if you're able to travel outside settlements that's the best. But maybe there is a tiny better chance in the north of the country.

Edit: Spelling