I haven’t been to many purely American Christmas parties where friends and family gather, but I don’t recall people continually singing carols during the evening.

That’s a Polish thing. Perhaps other cultures do it as well, but it’s a Polish thing for sure. Especially among expats.

I sit and smile during such sessions: I don’t know the words in their entirety (snatches here and there, perhaps a chorus), but I know the melody and am content just observing.