This extract may help: 

"[Once upon a time][1]": 

 > - is a stock phrase that has been used in some form since at least 1380 (according to the Oxford English Dictionary) in storytelling in the English language and has opened many oral narratives since 1600. These stories often then end with "and they all lived happily ever after," or, originally, "happily until their deaths." These are examples of the narrative form and occur most frequently in the narratives produced for children aged between 6 and 8.

>  -  It was commonly used in the original translations of the stories of Charles Perrault as a translation for the French "il était une fois", of Hans Christian Andersen as a translation for the Danish "der var engang", (literally "there was once"), **the Brothers Grimm as a translation for the German "es war einmal" (literally "it was once") and Joseph Jacobs in English translations and fairy tales.**


  [1]: http://en.m.wikipedia.org/wiki/Once_upon_a_time