1
general::
On some past occasion, as in I may have sung this piece once upon a time, but I don't really remember it. This phrase, first recorded in 1595, is frequently used as the opening line of fairy tales and stories told to children, as in Once upon a time there was a king who had three beautiful daughters.
American Heritage Idioms
2
general::
Phrase(s): once upon a time
Cliché once in the past. (A formula used to begin a fairy tale.) • Once upon a time, there were three bears. • Once upon a time, I had a puppy of my own.
McGrawhill's American Idioms And Phrasal Verbs