"I have an appointment in April."
Translation:Nisanda randevum var.
Because the word order is wrong and because you can't have the personal pronoun ben in the nominative case in sentences that are constructed with var. So, if you totally insist on using the (otherwise unnecessary) possessive pronoun here, the sentence should read like this: Nisanda benim randevum var.
If you want to say when something takes place in Turkish, is it that days of the week are always in the nominative case (e.g. pazartesi görüşürüz = see you on Monday) but months of the year are in the locative case (e.g. nisanda randevum var = I have an appointment in April)? Or is the locative here dependent on the 'var' construction? Or is there another explanation altogether?