"We are throwing a party the day after tomorrow."
Translation:Vi ska ha fest i övermorgon.
Well, it can't be grammatically correct to use the wrong gender. Everyone would understand you of course but unfortunately gender errors are disproportionately annoying to many native speakers. I mean, it really doesn't matter, as I said there's no risk of misunderstanding, but still for very many people it's just grates on their ears.
Aha, yes, you can add en if you want to. ha fest is one of those cases where verb + object sort of create a unit, it's like 'ha fest' is just an activity. But it's perfectly alright to say ha en fest too. It may feel like the party is somehow a little more separate, but it's just a nuance.