Formal:
* يوم واحد (yawm waahid): This is the most literal translation and is appropriate for formal settings.
Informal:
* يوم (yawm): This is a more casual way to say "one day" and is commonly used in everyday conversation.
* ب يوم (bi yawm): This is another informal option, meaning "in one day".
In the future:
* في يوم من الأيام (fi yawm min al-ayyam): This means "one day" in the future, referring to an unspecified day.
The best option for you will depend on the specific situation and the level of formality required.