The adverb someday means eventually or at some point in the future. So if you plan to visit Sri Lanka someday, you'd like to do it but you don't know exactly when it will happen.
Use the word someday when you can't be specific about when something will occur. If you're not sure when you'll next see your friend who's moving to Alaska, you can say, "I'll visit you someday soon!" And if you keep planning to make homemade bread but never get around to it, you might promise yourself you'll do it someday.