One day is a common phrase that refers to a future moment in time. However, there are various synonyms that can be used to express the same concept. For instance, the phrase "at some point" can be used to describe a time that is not specified but is expected to happen. Similarly, the words "eventually" and "someday" also indicate an uncertain time in the future. "In due time" suggests that something will happen at the appropriate time while "in the future" or "in time" denotes a general sense of when something might happen. Ultimately, there are various synonyms for "one day" that can help to add variety and nuance to our language.