You would use the imperfect Era to set the scene/begin the story and would use fue only for situations which indicated that some action had taken place at a specific time in the past.
Imperfect verbs set the scene which is interrupted by a preterite verb
For example: " Estaba Lloviendo cuando corrió de repente una chica en una iglesia'
= It was raining when a girl suddenly ran into (inside) a church
The imperfect is also used to indicate time and dates,describe people, feelings,and actions of unknown duration.
por ejemplo " Estábamos muy felices porque era una vacación muy devirtida "
= We were very happy because it was a very enjoyable holiday
For further details please seePralee's lesson: 2.10 Era v Fue in a story