"Once upon a time" 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.
Once upon a time is a stock phrase used to introduce a narrative of past events, typically in fairy tales and folk tales. It 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.