I added your answer "tendremos una..." as also correct. You can add the indefinite article. We are not including it in this instance because if you say "tendremos fiesta" you are talking in a more general way, without referring to a party in particular.
Sometimes we use very generic sentences without the article. Other examples would be for example:
No hay cerveza en la nevera.
There isn't any beer in the fridge.
Tengo que comprar coche pronto. Necesito uno para ir a trabajar.
I need to buy [a] car soon. I need one to go to work.
Deberías ahorrar dinero para la boda.
You should save [some] money for the wedding.
¿Tienes papel y lápiz?
Do you have [a] pen and paper?
Some of these are set expressions that we use with no need of an article.
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