Using fue or era

Question

Hola,I don't know if a reply occurred in some other area, but I was wondering if you could give a bit more infoto help me understand this.  I still have trouble with this particular example.My earlier question was following  your reply for : "Why the imperfect is used and not the preterit in:"puesto que era mi cumpleaños " (I sent this same request on my Questions and Answers page, and didn't see the reply there,)Muchas gracias, Nicole

Answer

Hola Nicole
This is the sentence you are referring to: 
"Yesterday my boyfriend gave me two lovely orchids since it was my birthday."
We can see that her birthday was "yesterday", so it is a block of time that is the past, not connected to today, the moment of speaking; this would make us think about using "fue mi cumpleaños" on the second clause (instead of era). That would be absolutely correct. However, if, despite referring to something that took place yesterday and it is a completed action, a person could use the imperfect "era" to convey more of a descriptive meaning/background information about the past (yesterday). This is precisely what this "era" here is doing. 
If we take that same sentence again and make up some more "descriptive and background information" we could say for example:

"Yesterday my boyfriend gave me two lovely orchids since it was my birthday and I turned 20; I had some friends at home for dinner..."
"Ayer mi novio me regaló dos orquídeas bonitas ya que era mi cumpleaños y cumplía 20 años; Tenía algunos amigos en casa para cenar.."

You can see that we can add some more descriptive sentences using the imperfect. They are all referring to "yesterday".
I hope this helped.
Saludos
Inma