Questions & Answers about Konserten er i september.
In Norwegian, the definite article is often attached to the end of the noun instead of being a separate word.
- konsert = concert
- konserten = the concert
So konserten already includes the meaning of the concert. That is why you do not say den konserten here unless you mean something more like that concert or you are using a special structure.
The dictionary form is konsert.
You would usually learn it as:
- en konsert = a concert
- konserten = the concert
So the sentence uses the definite singular form of the noun.