I have asked my teacher about articles and she gave me such an example:
A: Today we are going to a concert.
B: Okay! (the person does not want any details)
A: Where are we going now?
B: To a concert. I told you we are going to a concert today.
She said everything is correct here because the person I am talking to in the dialogue doesn't know what concert and I did not specify it but since she has been mistaken many times before, I want to ask you here. I would you "the" in "B: To a concert."