Refers to an unspecified future time; can also mean 'at some point' in the past, but the future sense is more common. The gloss 'the other day' is a less frequent past usage.
I hope to live abroad someday.
I want to go to Japan someday.
Let's meet again sometime.
I hope we can meet again someday.
何時 (いつ) is the question word 'when', while いつか (何時か) adds か to turn it into 'someday'; learners sometimes confuse the two because of the shared kanji.
Compound of いつ (when) and か (indefinite particle). The kanji spelling 何時か is a jukujikun reading.