fešta

1. colloq. party; get-together
   Spremamo feštu za njegov rođendan. — We're preparing a party for his birthday.
2. colloq. celebration; festival
   Posle pobede je u gradu bila velika fešta. — After the win there was a big celebration in town.

The word comes from the Adriatic coast (Croatia, Montenegro); in Serbia it is colloquial and often has a jokey, 'seaside' flavour. The usual words are žurka (party), proslava, slavlje (celebration).