Desi just said what I was going to - a "sing-song". My parents would put on a few in the course of the year - where fifteen or twenty couples would be invited, and my mother would play the piano - and then there would be any number of informal, unplanned sing-songs. That's the only term I heard anyone of their generation using; otherwise, it was just a 'visit' or a 'gathering' or a 'party'. I never heard of a gathering of instrumentalists in my younger days, other than having to do with formal bands of one kind of another.

This was in Canada. 'Way down South, they had a jubilee; the jokey folks, they had a jamboree.