Translation:April, June, September and November have thirty days.
“Aprilo, junio, septembro kaj novembro havas po tridek tagoj.” You don't use the accusative case after ‘po’.
I just copied the sentence and added "po", forgetting that it is a preposition. :)
Not an Esperanto expert here, but some personal experience may be of use...
I would think the latter. This construct looks Slavic to me, and, being a native Russian speaker, I've seen both. It's pretty much the difference between "X and Y have Z days" and "X and Y each have Z days." Only used with quantities, though.
I'm sure I could dive into the Esperanto grammar for a few hours and fish out chapter and verse, but this is the way it makes sense to me :)
Please feel free to slap me around a bit with a large trout if the above is incorrect.
Tridek tagoj en Septembro,
en April', Juni', Novembro;
Dudek ok en Februaro
(Dudek naŭ ĉe superjaro):
Restas sep monatoj, kiuj
Havas tridek unu ĉiuj.