one day; a certain day
Used to introduce a past event that happened on an unspecified day, often in narratives.
One day, he suddenly left town.
One afternoon, I met her in the park.
ある日 points to a specific but unnamed day in the past used to open a narrative, while いつか means 'someday' and refers to an unspecified day in the future.
Standard modern spelling.
Kanji spelling using 或る; less common but still seen.
Irregular okurigana usage; rarely used.
Combination of the prenominal adnominal ある (a certain) and 日 (day).