''Once upon a time'' is the firse traditional words said at the beginning of children's stories.

It should be "Once upon a time.." as a starting phrase in fairy tales, otherwise it should be "Once a day" which is usually being used in taking medications.

i think you mean 'Once upon a time'.. usually you;d us it in fairytales and it means something that happened a long time ago. usualmente se dice cuando comenza una historia que pasó hace mucho tiempo en el pasado. es muy común en los cuentos para los ninos