Hola Clara,

With or without the article the sentence is correct. You can say:

¡Qué bien que el sábado tendremos una fiesta! or

¡Qué bien que el sábado tendremos fiesta!

The second one where the article is omitted is talking about the more general meaning of having a party. It is pretty much a set phrase; ¡Hoy tenemos fiesta! ¡Hoy hay paella! ¡Pon sal en la comida!; here we are not being very specific. 

I hope this helped,

Inma