What would be the ideal translation of ‘one day’ and ‘day one’? I.e. you can choose to do something ‘one day’, or today could be ‘day one’ if you get started now.
A single day ("this task can be finished in one day") would be diēs ūnus/ūna. The first day ("I've been working since day one") would be diēs prīmus/prīma. Some unspecified day ("I'll get to it one day") would be diēs aliquis.
(The gender of diēs is a bit weird, as it can be either masculine or feminine. See this question for more explanation. Ūnus and prīmus look different in the masculine and feminine, but aliquis doesn't because it's nice like that.)