An expression meaning 'someday' or 'one of these days,' implying that something will occur at some unspecified time in the future (somewhat literary).
いつの日にかまた君に会えるだろう。
Someday I'll meet you again.
彼女はいつの日にか海外で暮らしたいと言っている。
She says she wants to live abroad someday.
いつの日にかこの山に登るのが私の夢だ。
Climbing this mountain someday is my dream.