"Are you having a party?"
Translation:Har ni fest?
As per Lundgren8 in the thread for the "translate into English" exercise:
”Ha fest” is sort of a semi-fixed expression, and then the article is often omitted. Compare ”spela piano” (play the piano), ”plocka svamp” (pick mushrooms), ”skriva brev” (write a letter), ”bygga hus” (build a house) etc.
Slightly longer source: https://forum.duolingo.com/comment/5764922/Har-ni-fest