ذَاتٌ مُرَّةٌ:. Once يَحْكِي أَنَّ:. It is said that. كَانَ يَا مَا كَانَ:. Once upon a time thank you Farid شكرا يا فريد

You simply can say ذات مرة

Do you mean how do we use 'once upon a time,' in a sentence? If so, then we usually use 'once upon a time,' at the beginning of children's stories or fairy tales. It means at some time in the past or some time ago. Example: "Once upon a time there was a princess..." It is a common way to start a story. This is the Persian translation I found online: kân yâ mâ kân fî qadîmi zzamân wsalifî al`aSri wal'awân... (كان يا ما كان،في قديم الزمان، وسالف العصر والأوان)