"Have a date" implies an existing or desired romantic relationship. It would usually be used if you were only meeting one person. "Have an appointment" is a somewhat formal term. You have an "appointment" with a doctor, for example. If you are going to be with friends, as a verb you would "get together with them." or 'hang out with them." It wouldn't matter what sexes are involved. I can't think of a good noun to describe this, other than maybe "meeting", which isn't quite right. Maybe someone else can help. By the way, "reunion" is a specific term related to a gathering of people with some relationship who haven't seem each other in some time.