1Refers to some unspecified point in time — either in the future ('someday') or in the past ('at some point, once'). Usually written in kana.
いつか日本に行きたい。
I want to go to Japan someday.
彼はいつか戻ってくるだろう。
He will return someday.
いつかこの問題の答えがわかる時が来ると思う。
I think a time will come when I'll understand the answer to this problem.
この写真、いつか撮ったのを覚えているけど、いつだったか思い出せない。
I remember taking this photo at some point, but I can't recall exactly when.