From context and my understanding, it's the semi-formalized entertaining of relative strangers in your home that they don't do. Dinner parties, living-room cocktail parties a la Mad Men, or wild (possibly teen) debauches — I think it's the "people you're not intimate with" part that's foreign. The size of Japanese homes probably contributes almost as much as the cultural reserve.
(It's not like wild parties don't happen, after all, they just happen someplace where you're not cleaning out your personal liquor cabinet.)
I went to the house party of someone who's really helped me out with my workHouse... Well... It's certainly a party at a houseBut when it comes to 'unique' ways of doing it, Helen's house party was by far the most surprisingHouse party!You could say thatBut! Whether you realize it or not, everyone here has house parties where they show their own unique 'hospitality'♪Like how we all came to Yuriko's house and watched anime the whole time... I think this is a house party tooAnd having an all night talk at Mika's house with Yui and Rina... I thought that was one tooWhoaaa, a Gal's Party! For us, that's like a different culture too...OMG-More than that, this amazing power of friendship you have... It's incredibleRight~Totally-I know right-♡Come!And then-Wha... Hold it Kate-san, isn't this seriously incredible!?
Cinderella Girls Theater
劇場第1375『色々な文化に飛び込みマス♪』
Theater Episode 1375 - Diving Into Different Cultures♪Just watch!!Is it like flower viewing...? That sort of characteristic thingFeels like a foreign culture thing... We don't really have those in JapanHelen's House Party!? What the heck, I wanna go... But it sounds a little scary!It gets really emotional 10 minutes from now-Huh, this is considered a house party too!?Get your handkerchiefs ready!