someday [also some day] :at a time in the future that is not yet known ‘将来某一天; 有朝一日’
例如:I hope you come and visit me someday/some day.
one day :used when you are talking about a time in the past or future without actually saying when 表示非明确的过去或将来的某段时间 ‘某一天’
例如:He came to see me one day last week.
We must go and visit them one day.
也就是说someday/some day 只能用于将来时；而one day 可用于过去时或将来时.将来是中二者可替换.