It would be nice to get clarification on this. ¿Dónde está la fiesta? does sound right. It could have to do with the fiesta being an event, as in, Where is the party occurring?
In our city, there is a Hispanic grocery chain called Fiesta. If I were to ask: "¿Dónde es la fiesta?"
I may get directed to that store. lol.. Seriously, for whatever reason, the people in my area randomly add " the " to the front of stores.
I need to go to the Krogers.
u_u instead of:
I need to go to Krogers.
I'm not sure it's random. I think "the" tends to get added when the word is not a proper Spanish noun. ¿Dónde está Miguel? ¿Dónde está el Mike? If I recall correctly, one doesn't add "the" before a proper Spanish noun, but I don't know if it's a rule to add "the" before a proper non-Spanish noun.
when talking about EVENTS (concert, baseball game, party, etc.) we use "ser" instead of "estar" even though this goes against the usual rule of "estar" for location