'Some day' and 'One day' are the same, you can use either one. Eg. Some day I'll travel to Japan Eg. One day I'll write a book They both indicate that there will be a definite starting point. In English, we say 'sometimes' to mean occasionally. But 'some time', we don't really say on it's own. Eg. I'll come back some time soon. Eg. I'll be gone for some time. But we wouldn't really say: 'some time I'll write a book'.