expression
someday; one of these days; one day
Expresses a hopeful or wistful expectation that something will happen at an unspecified future time. Often used in poetic or reflective contexts.
See also: いつの日か (いつのひか)
いつの日にか、また会えると信じている。
I believe we will meet again someday.
いつの日にか、世界が平和になることを願っている。
I hope that one day the world will be at peace.
いつか is the more common and casual way to say 'someday'. いつの日にか is more emphatic and literary, often carrying a stronger emotional or poetic nuance.
A fixed expression composed of いつ (when), の (possessive particle), 日 (day), に (particle), and か (question marker), literally 'on what day'. The phrase has become idiomatic for 'someday'.