To some extent you could probably use them interchangeably but they do carry a slightly different meaning. Some other day would imply that you have plans or had plans that you are putting off but intend to carry out such as... I was planning to go to the store today but I'll have to go some other day (next week). *Next week is not required, just using it as an example.* Maybe one day implies that you have no set plans (like a desire to go somewhere but have no plans or means to go) but would like to go. "I would like to visit Rome but I'm busy with work and college. Maybe one day when I have more time and money I can go." Maybe one day in that context expresses a desire to go but no definitive plans or means of going. Someone else here may be able to clarify better than I can. These would be a practical example of everyday speech or usage.