Name of Mountain in Paris

Hallo NG

I am looking for a mountain / Hill in Paris I have forgotten the name.
It is a Hill in Paris where people go in summer evenings. I think there is a 
church too. I was there in 1991 and loved it.
There were people with msical instruments (guitars, drums, ..) and they 
played and sang and partied. You can see Paris from this hill.

Can anyone give me the name of this hill please?
I have friends planning to go to Paris in a week, and I want them to go 
there, but don't know the name of this place.

Lotsa Love and thanks in advance!

Mario