"Am I seeing you tonight?"
Translation:Ser jeg dig i aften?
I'm also wondering why 'Ser jeg dig i natten?' is not correct. It obviously has a meaning different from 'Ser jeg dig i aften?', but from the English 'tonight' you can't really tell.
"Ser jeg dig i natten" means 'am i seeing you in the night' imho this is not about the following night.
Unfortunately I think they are just prepositions that you have to memorise when to use which! I've been learning German for a long time there doesn't generally seem to be an easy way of knowing which prep to use