I get 's avonds. As I understand it, it means "in the evening" in a habitual sense.
I have a problem with in de avond because the English translations are not without ambiguity. You could translate in de avond as "in the evening", but then you have a problem. You need to know if it is a particular evening ("In the evening we shall be dancing") or every evening in a set of planned evenings - on a cruise, say - (In the evening (or evenings) we shall be dancing"). That's ok for future events, but in the present we need more clarity. A one-off event demands "In the evening we dance (or are dancing)" If we want to use the present tense for this habitual activity, I contend that it should be "In the evenings we dance" using the plural to avoid ambiguity. With context, there would probably be no ambiguity, but without this context I think Duo should routinely allow the plural noun as a valid English translation.