What is a regular monthly meeting of witches when rituals are performed?

That is a very difficult question to answer, but here are a few (not all) answers.

Collective nouns for witches: Coven, convocation, circle, heath, hearth, grove, or gathering.

Other reasons why witches gather: celebration, rite, rite of passage, Sabbat, Esbat, ritual, coffee-klatch, study group, or gathering of friends.