Hola Tim

You could have used "era un día muy lindo" as well if the speaker was using this sentence with a more descriptive function instead of seeing it as a completed action. So, when using "fue un día muy lindo" you see this as "completed", you mentally place yourself at "the end of that day" instead of in the middle of it as if it was in progress. 

If there was no hint instructing you to use "el pretérito indefinido", the sentence with "era" would have also been accepted as if taking it more in a descriptive way instead. 

This lesson may show some light to it. The different nuances between the imperfect and the simple past are one of the difficult topics when learning Spanish because of the lack of differentiation in English. 

Saludos cordiales

Inma