one day

adverb
  1. 1

    At some unspecified time in the future.

    โ€œOne day I shall upgrade my software, but not just yet.โ€

  2. 2

    At some unspecified time in the past.

    โ€œOne day I was playing with a girl from my class.โ€

    syn:once

Source: https://en.wiktionary.org/wiki/one%20day