Rendez-vous when is(un, en, a) used
Hello people I have a question about using the (rendez-vous) word in french
I found some examples in the book that I am studying
1-)Ils ont un rendez-vous - This part I understood
2-)elle est en rendez-vous ---- but this part I did not understand, why is it (elle est en rendez-vous)? why can't it be (elle est en un rendez-vous)??? in the first explample ils ont un rendez-vous (un) was used, but why was (un) not used in the second example? this is confusing.
3-)Madame langlois a rendez-vous mercredi á midi - This part I did not understand why is it Madame langlois a rendez-vous mercredi á midi?? and why can't it be Madame langlois a un rendez-vous mercredi á midi, why was (un) not used in this sentence? why was it used in the first example but not in this one??
oui, il a un rendez-vous. - this part I understood
j'ai un rendez-vous mardi matin - this part I also understood
I am just confused about using the (un) in the sentence that has the word rendez-vous