"Dager og netter"
Translation:Days and nights
En natt - natten - netter - nettene A night - the night - nights - the nights It could also be feminine: "Ei natt - natta - netter - nettene", but that would be more "dialect".
Yes, I believe that's exactly the case. And certainly not the only one. Like en tann / tenner (also irregular in English).