"'s Ochtends of 's avonds?"
Translation:In the morning or in the evening?
Why not "Mornings or evenings"? I was thinking it was like if you were trying to make an appointment, and someone asked what time of day worked best for you.
Because ochtends and avonds aren't plurals. This construction is a fossilized remnant of the old case system Dutch used to have. The -s actually marks a declension of the noun. And the 's before the words is actually short for des, which is a definite article like de and het.
The case system has largely died out, but these are two of many such fossilized constructions.
(This is specifically the genitive case, if you're interested.)
"Ochtenden of avonden"?
Those are the plural nouns, but this doesn't make a whole lot of sense in Dutch.