Translation:We will have lunch together at twelve-thirty.
https://en.wikipedia.org/wiki/Esperanto_grammar#Prepositions "in Esperanto all prepositions govern the nominative. The only exception is when there are two or more prepositions and one is replaced by the accusative."
For the evening meal, the Esperanto course seems to prefer literally saying ‘the evening meal’, and they accept both ‘dinner’ and ‘supper’ (and reportedly also ‘tea’) in translations. So they really ought to accept ‘dinner’ (in addition to ‘lunch’) in translation for the midday meal too.