"Dinner is ready."
Translation:Aftensmaden er færdig.
See my reply above. Basically it's because it is referring to a specific dinner. You could use another expression like 'Nu er der aftensmad' or 'Så er der aftensmad' (Now there is dinner) and it would be correct. But of you added 'klar' (ready) after any of the two sentences, then you would again have to use aftensmaden, as you would again talk about the specific meal (Nu er aftensmadEN klar). Hope this makes sense?