Could this also mean "We will eat on the twentieth (day of the month)?" Or would that question use a different preposition?
That was what I wondered, too. And how would I know whether it's the 20th of the month or 20:00?
And I wondered the exact same thing as well!
It's pretty clear from context what is meant. Since it's taking about eating, it's likely that hours is meant, and not days, or even floors.
We have to use some assumptions in English, too. If I said "We will have dinner on the 20th" it could mean April 20th, May 20th, the 20th of some other agreed-upon month, or even the 20th floor of a building.
In English, it doesn't necessarily have exactly one meaning when it's taken out-of-context, and neither does it in Esperanto.
But why are we supposed to translate it to a 12 hour system? We eat at 20:00 is perfectly understandable.
Yeah, I got it wrong because I left it in 24 hour time too... I guess just roll with it and know different next time. :(