expression, noun
one day; a certain day
Used to introduce a past event that happened on an unspecified day, often in narratives.
ある日、彼は突然町を去った。
One day, he suddenly left town.
ある日の午後、私は公園で彼女に会った。
One afternoon, I met her in the park.
いつか refers to an unspecified future day ('someday'), while ある日 refers to a specific but unnamed day in the past or in a story.
Combination of the prenominal adnominal ある (a certain) and 日 (day).