one day, 24 hours / the daytime, the period from dawn until dusk / (figuratively) a short period of time / one day, someday, an unspecified day / the first day of the month

one day

(formal) first-person singular pronoun: I, me

a boy, a young man

(rare) the first day of the month

one, 1 / the first in order, foremost / the beginning / the best / a kind of; one of something / (card games) an ace / unity / the bottom string of a shamisen or other Japanese musical instrument

excursion, school trip

being from a certain place / coming out

to go on an excursion

私: I; first person pronoun