You're right. एक न एक means one or the other. एक न एक दिन means one or the other day. It can be used with other nouns. For ex - एक न एक दिन मैं जीतूंगा means "I will win one day or the other." एक न एक शाम मैं आऊंगा means "I will come one evening or the other." एक न एक बच्चा बड़ा बनेगा means "One or the other (or at least one) kid will earn a name for himself" एक न एक पीटेगा means "Someone will get beaten"