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.
いつか日本に行ってみたい。
I want to go to Japan someday.
I hope to live abroad someday.
Let's meet again sometime.
いつかまた会えるといいですね。
I hope we can meet again someday.
いずれ is a slightly more formal way of saying 'eventually/at some point', often implying it is bound to happen, whereas いつか is more casual and open-ended.
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.
sooner or later; in due time; eventually
before one knows it; unnoticed; unawares
Compound of いつ (when) and か (indefinite particle). The kanji spelling 何時か is a jukujikun reading.