@bumtildeath - "There are no parties tomorrow" should be correct.
Hola bumtildeath. I am very curious as to where this answer is accepted as correct. I don't think that it should be. Here's why not.
The example exercise sentence is, "Las fiestas no son mañana." As I understand it, this is a simple present tense declarative statement with a plural feminine subject, a negator thrown in on the present tense verb "son".
"There are no parties tomorrow" seem to use different parts of speech which we have not been shown by duo yet. I think "There are" is an existential clause.
The word "no" in the phrase "no parties tomorrow" negates the word "parties" instead of the verb "are".
Personally, I think the grammatical structure of this translation is beyond the scope of what we have been shown so far and the accuracy of the translation has suffered as well.
We know from the phrase "Las fiestas" in the root language exercise sentence that the definite article "Las" in referring to some known "fiestas". But in the sentence "There are no parties tomorrow.", the notion of the known specific partys is replace by a statement regarding the lack of parties in general.
A user by the name of Lago has consistently pointed out this types of problems then the translation into the target language doesn't not preserve the grammar of the root language and the impact it can have on the accuracy of the translation.
He's definitely worth following if you want to tighten up your translation skills.
Adios amigo :)
In English I'd say, "Monday is a holiday." Not every Monday, just this Monday. But I'd say, "Monday there is a holiday party at the park." Not every Monday, just this Monday. In the former, "is" would be like "ser," as you indicated; whereas the latter use of "is" indicates a temporary state and "estar" would be appropriate. Hence, Duo and you are correct in my estimation. I'd just add that "estar" indicates the temporary, non-permanent state.