This is key from BBC Learning English.

And today I'm going to tell you about some informal ways of using English now in English.

When we have scheduled formal meetings, we can say we have an appointment.

So, for example, I can say I have an appointment with my doctor or I have an appointment with my lawyer.

But you never, ever say I haven't with my friends.

That's too formal now to talk about appointments with friends.

We simply say, I'm meeting my friends this weekend.

So is meet someone, Have an appointment, remember, beat someone and I'll meet you next week.