one fine day—(also: one day)
1. at some distant time in the future (used as a promise or prophecy):
- Neither was I to know that I should one fine day have a son who would make me very proud of him.
2. on a certain day in the past:
- But one fine day in early summer … Maxine found herself canceling an arrangement to meet a friend in town.
one of these fine days —(also: one of these days) at some time in the future that may be quite soon (used as a promise or prophecy):
- One of these fine days he’ll realize what a fool he’s been.