Hola,
Here is my train of thought. "Ayer estuve en un concierto:" "Yesterday" requires the pretérito indefinido because it refers to a completed action at a time in the past, and the verb is estar because it is referring to a location; hence estuve.
However I am stuck with "Fue en San Juan:" Is San Juan not a location? If so, would it require the verb estar rather than ser?
It would be great if you could explain this.
Many thanks.